Consuming riba is a major sin and one of the serious sins that incur punishment. Allah says (interpretation of the meaning): “O you who believe! Fear Allaah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers.
279. And if you do not do it, then take a notice of war from Allaah and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)”
And the Messenger of Allaah (peace and blessings of Allaah be upon him) cursed the one who consumes riba and the one who pays it, the one who writes it down and the two who witness it, and he said: they are all the same. Narrated by Muslim 1598.
But dealing with a person who consumes riba is permissible. The Prophet (blessings and peace of Allah be upon him) dealt with the Jews although they consumed riba. What is haraam with regard to the wealth of one who consumes riba is the extra money obtained through riba only; the rest of his wealth -- if it is from a halaal source -- is halaal.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) was asked about those whose wealth is mostly haraam such as tax collectors (referring to the maks, a haraam levy or tax), those who consume riba and so on, as well as those who practice haraam professions such as those who make images of animate beings, astrologers and the helpers of oppressive rulers. Is it permissible to buy food from them or not?
Praise be to Allah. If their wealth consists of both halaal and haraam, then dealing with them is a grey area; it cannot be ruled to be haraam unless it is known that he is giving him something that it is haraam to give, and it cannot be ruled to be halaal unless it is known that he is giving him something halaal. If most of it is halaal, then it cannot be ruled that it is haraam to interact with him; if most of it is haraam, it was said that it is halaal to interact with him and it was said that rather it is haraam.
With regard to the one who deals with riba, the majority of his wealth may be halaal, unless it is known that it is not halaal for some other reason. That is because when he sells 1000 for 1200, it is only the additional amount that is haraam.
If his wealth consists of both halaal and haraam and they are mixed, the halaal part of it cannot be ruled to be haraam; rather he may take the proportion that is halaal, as in the case where there are two partners and the wealth of one of them is mixed with the wealth of the other, then it should be divided between the two partners. The same applies with regard to the one whose wealth is mixed between halaal and haraam; the portion that is haraam should be taken out and the rest is halaal for him. And Allah knows best. End quote.
Majmoo’ al-Fataawa, 29/272-273
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) favoured the view that wealth that is acquired by haraam means, such as riba, earnings of a female singer, selling drugs and bribes is only haraam for the one who acquires it; as for the one who takes it from him for a permissible reason, such as if he spends it on his children and his wife, or on a gift or payment for work and so on, it is halaal for the one who takes it.
He (may Allah have mercy on him) said:
Everything that is acquired by haraam means is haraam and does not benefit the one who acquired it, and if he spends it he will not be blessed in it and if he gives it in charity it will not be accepted from him, and if he leaves it behind, it will be his provision for the Hell fire.
But with regard to the one to whom he gives that thing, it is not haraam for him, because the prohibition is connected to the way in which it was acquired, not to the wealth itself. Everything that is haraam because of the way in which it was acquired is only haraam for the one who acquired it in that way, but for the one who takes it by permissible means from the one who acquired it, it is not haraam for him. But as for that which is haraam in and of itself [such as stolen wealth or wealth acquired by force], it is haraam both for the one who acquires it and for others. End quote.
Fataawa Noor ‘ala al-Darb, 13/179
He also said:
There is nothing wrong with a person accepting zakaah or donations from riba-based banks, because he is receiving it by lawful means, and the sin is on the bank for acquiring it by means of riba. The evidence for that is the fact that the Messenger (blessings and peace of Allah be upon him) accepted gifts from the Jews and he accepted invitations from the Jews, although it is well known that the Jews consumed riba and acquired wealth by unlawful means.
Similarly he interacted with the Jews in Khaybar, i.e., he gave them some of the palm trees and crops on the basis that they would have half and the Muslims would have half, although it was well known that they consumed riba.
Hence you may follow a useful principle: If someone acquires something by any permissible means then the sin of the one who acquired it by haraam means is not passed to him, because it is haraam because of the way in which it was acquired, so it is haraam for the one who acquired it in an inappropriate manner. But if you know that this wealth is the property of so and so, such as if a thief stole a sheep and brought it to you as a gift, then it is not permissible for you to accept it from him, because you know that this sheep belongs to So and so. But if something is haraam because of the way in which it was acquired, it is permissible for the one who takes it by lawful means. End quote.
Liqa’ al-Baab al-Maftooh, 27/140
Based on that, there is nothing wrong with proposing marriage to this girl, if she is religiously committed and of good character, and it does not matter that her father puts his money in riba-based banks and will supply his daughter’s trousseau from this wealth. The sin of that haraam wealth is on her father only, and there is no sin on her or on you because of that, in sha Allah.
You have to advise her father and tell him not to do this evil thing; you should explain that riba is devoid of blessing and it will be a cause of loss and shame on the Day of Resurrection.
And Allah knows best.