To respond to the original inquiry - I think the guest behaved entirely appropriately.

Food was PUT on her plate - she didn't take too much and then fail to eat it. She ate as much as she could, leaving the parts she didn't like/didn't want. Bell peppers are one of the few foods I have trouble choking down - they trigger a very strong gag reflex, and I get very sick to my stomach if I eat them. I likely would have done the same, and move them around/left them on the side of the plate.

I was also surprised at the comment about how much desert she ate - she enjoyed it, there was plenty. She's apparently fit and generally healthy - why is that a big deal?

When asked about my own dietary restrictions - no beef or pork (generally moral reasons) or bell peppers, I will generally cheerfully offer to supplement dinner (particularly if it causes a problem). One of the toughest situations I have been put in lately was being served a plate of veal at a wedding reception. I ate as much as I could without become physically ill, and said nothing. I would have been somewhat embarassed to be asked about it.

Close friends - I might say something like "I was thinking of taking that dish to a dinner party - what do you think?" Most people that come to my home I'm close enough to to be able to have a frank conversation beforehand about their preferences, and allow them to control what/how much they take. Hasn't been an issue at all.