I have gone out many times with rain pants and never put them on. I have three pairs, one for SAR and then some Precips and also the REI Event pants. I think I have used one or two of them maybe once. He should top half raingear like a jacket or maybe a poncho. I would get some of those two dollar emergency ponchos, stuff them in the side pockets of his pack without him knowing, and then congratulate him when he shows up properly prepared. But I am a wisea$$ and I am intolerant of fools, after being lied to ("I have been backpacking many times" and they cannot hike two miles, have Big 5 junk and can't seem to figure out how their own stuff works) and sometimes ending a trip with my pack plus theirs.

Sell him on a poncho and tell him to carry it or find someone else to take him out and risk sitting at a fire trying to keep him warm instead of hiking out on time.