What should a mother do with her adult son who used to be committed to prayer, but then started feigning prayer secretly without actually praying? Is she sinful for this, especially given the father's absence due to illness?

If you have raised your son to pray and advised him, you bear no sin if he later becomes negligent, for Allah the Almighty says: "And every soul earns not [anything] except for itself, and no bearer of burdens will bear the burden of another." The father has an important role in rectifying his son's behavior; so let him take him to the mosque if he can, otherwise, let him lead you in prayer at home so that you may both attain the reward of congregational prayer and you can be assured of his prayer. And know that supplicating for his guidance and righteousness has a great impact.