In my house, my DC have always understood that the same rules don't apply to guests.
They know they must eat their salad/veg but that I will not insist for guests because they are err....guests Four kids aged 5-17 and this approach has never caused me any problems.
Desert is not a regular event in this house, people can have fruit, yogurt etc, occasionally ice cream or cake yum,but if I have planned to give them desert everyone gets one, it's not conditional on eating the main, it's only food!
Bit of a digression, but I do get a few problems with children who apparently CAN'T AT ALL drink water.
Sometimes we have juice (sometimes lemonade!!), mostly we don't because I don't consider it essential and don't always buy it.
Children who tell me they can't drink water are offered milk, then sympathy if they still insist They always drink the water in the end.
(yes, even you DD's friend who is 11 who tried to tell me she was allergic to water!!(grovelling apologies if anyone really is allergic to water btw !!!!!!!!!!)
I tend to generally be sympathetic to faddy visitors, and always offer bread or toast if they don't like the meal I've cooked.
This sympathy stems from dealing with my own DS1 (now 17) used to be unable to tolerate food that was mixed up, or touching, or mashed, or slimy, or red or icy or fizzy or spicy or squashy or yellow or with a sauce or gravy .. ....(Laydee turns into a quivering wreck remembering how trying he used to be ....He still couldn't force down mashed potato to this day!
PS veggie sausage pasta with broccoli peas and sweetcorn rocks!