Denice, one idea...
I'll agree that while it's not fun when people impose their limitations it's actually a very useful skill to have, the ability to meet such requests, because if you ever find yourself cooking for someone who's seriously ill many of the same skills are incredibly helpful.
I'd just try to stick with simply prepared meat or fish - but - there's no reason why it has to be boring for you! So how about using flavored oils & flavored vinegars? That way, I can steam, bake, roast, grill either meat or fish (or veggies), offer it plain or relatively plain, with the flavorings on the side. In addition flavored butters (aka compound butters) and most sauces can be offered on the side. Yes, it does go back to very simply prepared food, but it doesn't mean that you have to eat the 'plain' version (and if they see you really enjoying your food they might try something new).
I originally started with Michael Chiarello's slim paperbacks on flavored oils & vinegars - but - there are SO many recipes on the web for making these, plus a million sauces, plus flavored butters too, that you really don't need a book... more your own imagination and tastebuds to guide you. Think: roast chicken with tzaziki (aka tzajiki), grilled steak with a bleu cheese butter or cabernet sauce on the side, roast salmon with ginger and lemongrass oil?
Best wishes,
R.