Your H is handling this well, but the only thing I might suggest is trying to call her more often if you have time. I know some people (my grandmother!) who are just "phone" people and all they want to do is chat, chat, chat.
For slightly anti-social me, it's easier to call them rather than having to actually go out with them and be at their mercy for length of time spent with them. A phone call is fifteen minutes once or twice a week.
And then you say, "Oh, look at the time, it''s time for dinner/TV show/bed/shower!" Such an easy out.
It doesn't sound like you have any problems with MIL per se, so maybe the frequent phone calls would help.
My suggestion though is to never feel obligated to spend time with MIL outside of family events. I don't think I've ever hung out with MIL one on one. I've hung out with her and SIL for a "girls" thing and just me and DH and her, but not like a shopping date or anything.. eep, that would make me feel awkward.