Is it permissible for a mother to ask her son to divorce his wife, whom he married while she was still married to her deceased former husband, and who used to send him her pictures and deal with magic and sorcerers, knowing that the son refuses to divorce her?

The duty was to forbid your son from the forbidden relationship, to help him reform, and to marry him off.

If the wife has repented with sincere repentance, then do not seek her divorce, for repentance erases what came before it, and striving to separate between spouses without justification is a severe sin.

However, if the woman has not repented, then it is correct to ask your son to divorce her. Divorce may be obligatory in this case if the wife is not chaste or is negligent in Allah's rights. If your son refuses to divorce her, then there is no blame upon you.