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.