I've only skimmed through this, but I feel I can/should comment as a heavy rider myself.
I always dread that my weight will negatively impact on a horse. In fact, only recently my friends had to work hard to convince me to have a short walk/trot ride on one of their horses, a 14-15HH Quarter Pony because I kept insisting that I would squash him (which proves, by 'the majority of fat people realize just how fat they are, and most of them get rabid if you point out that they need to lose weight.' that I'm in the minority!).
OP, if you are unsure about being direct, bring up your mares health problems. Say that you're worried about her health and that you want to cut her back to light riding.
But 'direct' would be best. It all really depends on how she feels about her weight. Me, someone could come out and tell me I'm too fat and I wouldn't really give a **** (though it wasn't the case a few years ago). But if she is sensitive, you will need to be careful. I'd say definitely still let her do groundwork, as it could also make good exercise for her to help her out a little.
It's a shame that you are at such a good weight yourself really, as I know that I am currently in a good place as I have a friend that would like to lose some weight, so she and I are working together to shift the kilos. Just an idea though, depending on how fit she is (because fit and fat aren't always polar opposites), maybe see if she wants to go trail riding on foot, as in you ride and she walks. Compan can really help motivate exercise.