As precautionary measure, should clean all the toys. Those that can soaked in water should soak in dettol (like lego). Those that cant, at least give them a good wipe with damp cloth (add some dettol).
All preschools will also do scrub down when they discover a case in their school, so I think the same should be done for your mother in law's place, esp the toys.
To safeguard your son, it's good to try to boost his immune system. I give my son Sambucol since he started playgroup and when he kena HFMD from church, we continued to give him Sambucol and his HFMD was very mild (no discomfort or loss of appetite) and the doctor said he recover very fast.