my sister used when she was cooking and going through a vegitarian phase, or my dad when he gave up meat for lent-he would have to cook something with meat in for us lol
I was a strict veggie for years, still don't eat much in the way of meat - I don't have guests round for meals (no one I know is suicidal enough to let me cook for them), but mr mac and mini me both eat meat and I do all the cooking (badly).
I would think it depends on why they're vegetarian.
If they just don't like the taste of meat, they may prepare meat dishes for guests.
I suspect those who are vegetarian because of ethics/treatment of animals/animal being killed for food/etc, would have much more of a problem cooking it.
I used to have a mate/housemate who was veggie while I was at uni and I would cook (badly) for us both, I was vitually a veggie too just because it was easier. I don't think I could do it again though, I like my meat and fish too much these days. However I would never knowingly invite a vegan in to my home... I just wouldn't be ar$ed, (sorry if that's offensive to vegans out there, it's just my own personal feelings on the matter).