I run into similar (but not as bad) problems when i visit my dad. You know - the food is always good, and not necessarily junky (sometimes from trader joe's, but canned/bottled) but they get barely any fiber in their diet, where we're USED to a lot, so I always have pooping issues for the whole time and then a few days after, and just generally feel yucky.
They also exclusively drink diet soda... and always try and pick some other flavors of diet up for us, which is just a big "NO" on the aspartame, for us, but they don't get it. And then they send us home with piles of food that we don't necessarily want because of the other issues... Meh.
They also insist on cooking when they come to our house *sigh*... which is so nice, but they think we're crazy when I choose the organic chicken or organic milk or whatever at the store. They think we vastly overpay for food. I think they're paying in their health (at least that seems pretty obvious to me, considering their health situation...), but what do I know, I'm not a doctor
(they can totally afford better food - she's a lawyer and he's higher up in the government... they definitely make a lot more than we do, and have no kids at home and fewer other costs)
I feel your pain, I guess, OP. Aspartame is a big struggle for me with my dad and his wife.
But then, I'm a total foodie and clearly a food snob, so I guess there's that aspect.
I think in this situation, I'd have to go with bringing some nice maple syrup and holding my breath through the rest. Offering to cook is really nice, but speaking personally, I really HATE when other people are trying to cook in my kitchen. It makes me crazy. LOL I assume most other people don't feel that way, though, but I'd have a hard time offering because of my own hang-ups with it.