She took some of her husband’s money without his knowledge to spend on her son’s marriage

Firstly: 

The father is obliged to help his son remain
chaste by means of marriage, especially if he asks him to do that and he has
the need and desire for it, so as to protect him from temptation and to
maintain his chastity. 

Please see the answers to questions no.
83191 and
87983 

Secondly: 

One should be moderate with regard to
marriage expenses and not go beyond the bounds of moderation; the spending
should not reach the level of extravagance and wastefulness as is common
among people nowadays. 

The scholars of the Standing Committee
said: 

Your spending half a million riyals on your
daughter’s marriage comes under the heading of extravagance that is
forbidden and there is the fear that you may incur punishment because of
that unless you repent to Allah, may He be exalted, and give up this
extravagance, because wealth belongs to Allah, may He be exalted, and people
are entrusted with it. Islam has set out guidelines on how to handle this
wealth and has forbidden extravagance and wastefulness. Allah, may He be
exalted, says (interpretation of the meaning): “And those, who, when they
spend, are neither extravagant nor miserly, but hold a medium (way) between
those (extremes)” [al-Furqan 25:67].

What is meant is: do not be extravagant by
overstepping the limits of generosity and spending on sinful things, and do
not be miserly in spending; Rather be in between that, i.e., between
extravagance and miserliness; adopt a middle path.

End quote from Fatawa al-Lajnah
ad-Daimah, 16/220-221 

Thirdly: 

It is not permissible for a woman to spend
her husband’s wealth except with his permission, unless he is stingy towards
her and her children and does not fulfil his duty towards Allah by spending
on their maintenance as obliged. In that case it is permissible for her to
take from his wealth whatever is sufficient for her and her children, on a
reasonable basis. 

Please see the answer to question no.
150250 

If the husband has not been miserly towards
his son with regard to his marriage and has spent on him in a reasonable
manner, without being stingy but in accordance with his financial situation,
it is not permissible for you to take anything from his wealth, even if it
is to spend on your son’s marriage. 

You mentioned that you had spent a great
deal on this marriage. If what had been spent was sufficient for what is
customary for someone like your son, then he does not have the right to
anything more than that and what you took was a transgression against your
husband’s wealth. 

If it was in line with what is usually spent
on marriage in your country, for people of similar standing to you, then you
have the right to make up the costs for your son’s marriage in a way that
will not adversely affect his father’s wealth or be unfair to him. 

What you must do is pay attention to the
rights of Allah to this wealth and to the rights of your husband who is the
owner of this wealth, as well as the sanctity of the oath that he swore and
what is in the best interests of your other children. 

You also have to repent and ask for
forgiveness, and you have to tell your husband what you did and ask him to
forgive you and let you off. 

But if you think it most likely that your
relationship will be adversely affected or that your husband will get angry
and there will be problems between you, then you do not have to tell him.
With regard to the extra money that you spent, if you have money of your own
from a salary, inheritance or the like, then you should pay back from your
wealth what you took from your husband’s wealth without his permission. 

If you do not have any wealth, then repent
to Allah and ask for His forgiveness for that, and strive to treat your
husband kindly as much as you can and pay attention to his rights, in the
hope that Allah will forgive you and set things straight between you. 

And Allah knows best.