So for the first time in years, I read Dear Abby. A reader called "Some like it Hot" writes
"My wife accuses me of "acting pretentious" when we are dinner guests at a friends's home and I warm my plate in the microwave. What am I supposed to do when the food has gone cold or lukewarm and I want my meal hot?"
Abby responds that she likes hot food too and to handle it by asking the hostess if it is ok to take it into the kitchen to zap.
I'm sorry, but this is just rude. Suck it up. Unless this guy is eating weekly or daily there, don't insult your host by telling them the food is too cold for your taste.
Of course, they could be really, really good friends and this is sort of a compliment that you feel comfortable enough to be 'at home' at their table.
Still, there are better ways to handle it, IMO.
When these people are guests in your home, serve piping hot dishes and if one starts to cool mention how you dislike lukewarm food, apoligize and heat it up.
Or just bring up food temperature preferances in general conversation, not during dinner. Complain about a restaurant that served lukewarm food ... or how embarassed you were when a dish you served went lukewarm ... how you hate buffets because the food is too cold ... something. A savy person AND friend will pick up on this and make an effort to serve a meal that is warm enough for you. If not ... suck it up.