There is a young man and there is a girl whom he wants to marry, and the two families agree. But before they got married he touched her breasts (sorry for saying that, but it is in order to explain the matter) and he put his hand and his head and his mouth on her chest… They want to repent from this sin. The young man is also experiencing strange thoughts that may be waswaas from the shaytaan, and we seek refuge with Allaah from the accursed shaytaan. These strange thoughts include the notion that because he touched this girl’s breasts, she is haraam for him, especially since that happened when there was some talk about breastfeeding an adult. I have read this hadeeth and the reasons for it, and I have also read many other ahaadeeth, which say that there is no breastfeeding after the age of two years or after weaning. But the shaytaan comes at him from all places. Now he wants to marry this girl but the shaytaan is whispering to him that this marriage is haraam. What should I do?.
It is not permissible for a fiancé to touch his fiancée, or be alone with her, or kiss her, because he is a stranger to her (non-mahram) until the marriage contract is done. It is only permissible for him to look at her when proposing marriage so that he will marry her knowing about her, and this could help to make the marriage successful.
Based on this, what you mention about touching the breasts etc is haraam and he has to repent from that, as well as regretting it, praying for forgiveness and resolving never to do that again.
Each of them also has to keep away from things that may lead to temptation and things that may tempt them to do haraam things, and they should be keen for their married life not to start with disobedience towards Allaah, may He be exalted.
The things mentioned do not mean that the girl becomes haraam for him (a mahram), even if we assume that he breastfed from her, because breastfeeding of an adult does not have any effect, according to the majority of scholars. See also question no. 2864.
The issue referred to in the question has to do with sucking the breast and drinking milk, not just kissing or sucking without milk being produced.
To sum up: all they have to do is repent to Allaah, may He be exalted, and they should hasten to get married if possible.
The best treatment for waswaas is to remember Allaah constantly (dhikr) and to offer prayers regularly on time with the congregation, and to adhere to obedience to Allaah, may He be exalted. Then one should ignore the waswasah and not pay any attention to it.
And Allaah knows best.