Praise be to Allah
With regard to washing the deceased, it is stipulated that the water should reach all parts of the body, by analogy with ghusl for janaabah in the case of the living.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
What is obligatory in this case is two things: the intention, because it is an act of purification that is done as an act of worship, and is akin to ghusl for janaabah; and making the water reach every part of the body, because it is ghusl, in which that is required, as in the case of ghusl for janaabah.
In the case of washing the deceased, three things are stipulated… The second condition is that the water must reach every part of the body, because the Prophet (blessings and peace of Allah be upon him) said: “Wash him with water and lotus leaves.” And because he (blessings and peace of Allah be upon him) said to the women who washed his daughter: “Wash her.” If the author had quoted this as evidence, that would have been better than quoting as evidence analogy with ghusl for janaabah, because this is a clear text: “Wash him…” which includes the entire body, as does the command “Wash her.”
End quote from Ta‘leeqaat Ibn ‘Uthaymeen ‘ala al-Kaafi li Ibn Qudaamah.
The scholars (may Allah have mercy on them) stated that if there is something adhering to the body of the deceased, then it must be removed, so that ghusl may be done.
Al-Bahooti (may Allah have mercy on him) said:
Poultices on wounds should be removed, as should any bandages, in the case of obligatory ghusl, and the area beneath them should be washed, so as to make the water reach all parts of the body in ghusl, as in the case of one who is alive.
Khashshaaf al-Qinaa‘ (4/414). End quote.
Based on the above, if that nail polish forms a layer that prevents the water reaching the body, then it should be removed from that woman by whatever means, even if it is a chemical substance, on condition that the deceased will not be harmed by the substance that is used to remove the nail polish.
For more information, please see the answer to question no. 122678
And Allah knows best.