Praise be to Allaah.
There is no blame or sin on him if he gave her the choice and
she chose to stay, rather the blame and sin is on his mother who is forcing
him to do this. If he can advise his mother himself, or through the
mediation of someone whom she will listen to, and tell her that it is not
permissible for her to do this, and make her fear punishment in this world
or in the Hereafter, then this is what he must do, otherwise Allaah does not
burden any soul beyond its scope. End quote.