Depending on MIL's age and other factors, and how she is to get along with normally, this may have been a slip of the tongue, that she wishes wasn't said...

IMO often sons have trouble "talking" things like this over with their mother...

Sometimes a simple comment at the time of offense is -"I'm sorry YOU feel that way..." but he hasn't complained to me".. or some such whatever fits...

But seems like he could still golf if he wanted to, but I know it is costly.