Serious Eats - seriouseats.com

of course vegetarians can survive a meal without protein, but having a separate vegetarian entree is a nice gesture. a person with a restricted diet (who may feel used to feeling like a second class dinner party guest confined to side dishes) it is likely to be very appreciative of the effort.

anyway, if your friends eat fish you could make both a mild fish and a chicken main course. you could cook them with the same method for ease and prepare them with similar seasonings, so that either entree will go fine with similar side dishes and wine. you wife eats the chicken, the fishetarians eat the fish and the remaining guests can chose between the two. some may even choose both.

for non fish eating veggies, you could do the exact same with tofu or tempeh (or portabella if the meat dish is beef).

i'd suggest having at least 2 or 3 side dishes (everyone will find one they’ll eat) and serving it family style.