Is the son of her husband’s daughter regarded as a mahram for her?

Firstly: The son who is asked about here is regarded as a mahram for this woman, so she may shake hands with him and uncover in front of him as she uncovers in front of her mahrams, and it makes no deference that his grandfather is deceased. 

The way in which he is her mahram is that she is the wife of his grandfather on his mother’s side. Permanently forbidden in marriage for a man are his father’s and his grandfather’s wife, no matter how far the line of ascent reaches, whether the grandfather is on his father’s side or his mother’s, because Allaah says (interpretation of the meaning): 

“And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way”

[al-Nisa’ 4:22]

It says in Zaad al-Mustaqni’: The marriage contract makes the wife of the father and every grandfather a mahram. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in his Sharh: The marriage contract is valid, and makes forbidden in marriage the wife of his father or grandfather, no matter how far the line of ascent reaches. Every woman who has been married by his father, even if he divorced her, is permanently forbidden in marriage to him, and the same applies to every woman who was married by his grandfather, whether on his father’s side or his mother’s. The evidence for that is the verse in which Allaah says (interpretation of the meaning): 

“And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way”

[al-Nisa’ 4:22]

If he marries her, that is worse than zina, because Allaah says concerning zina (interpretation of the meaning): 

“And come not near to unlawful sex. Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin, and an evil way (that leads one to hell unless Allaah Forgives him)”

[al-Isra’ 17:32]

But concerning this, Allaah says: “indeed it was shameful and most hateful, and an evil way”. Marrying mahrams – Allaah forbid – is worse than zina. Hence many of the scholars were of the view that the one who commits zina with one of his mahrams, even if he is not married, is to be killed. There is a hadeeth concerning that in al-Sunan. So the wife of one’s father or grandfather, not matter how far the line of ascent reaches, is forbidden in marriage, whether on the father’s side or the mother’s, and Allaah has not stipulated that the marriage should be consummated etc, rather this ruling comes into effect by virtue of a valid marriage contract. End quote from al-Sharh al-Mumti’, 5/198 

This is a matter concerning which there is no difference of opinion among the scholars. Ibn Qudaamah (may Allaah have mercy on him) said, discussing the fact that a father’s wife is a mahram: It is the same whether she is the wife of his father, or the wife of his paternal grandfather, or the wife of his maternal grandfather, whether near or far.  There is no dispute concerning that among the scholars as far as we know, praise be to Allaah. End quote from al-Mughni, 9/518, 524 

Secondly: 

With regard to kissing, she may kiss him so long as there is no risk of fitnah, and it is better to kiss him on the forehead or head. 

Imam Ahmad (may Allaah have mercy on him) was asked about a kissing a mahram man. He said: If he has come from a journey and there is no fear (of fitnah). But it should never be on the lips, only on the forehead or head. 

Al-Adaab al-Shar’iyyah by Ibn Muflih, 2/266 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about kissing mahrams. He said: 

If kissing mahrams is done with desire or one fears that desire may be provoked, it is haraam without a doubt. If there is no fear of fitnah then there is nothing wrong with kissing on the head or forehead. As for kissing on the cheek or lips, that should be avoided, except in the case of a father and daughter or mother and son, for example. It is more lenient in this case because Abu Bakr (may Allaah be pleased with him) entered upon ‘Aa’ishah (may Allaah be pleased with her) when she was sick and kissed her on the cheek, and said, “How are you, O my daughter?” End quote, from Fataawa ‘Ulama’ al-Balad al-Haraam, p. 691. 

And Allaah knows best.