well you have a couple of options

the first is dont go with him

aside from that talk to him and decide how much you care

I survived hiking for many years in jeans. they suck when wet but that alone won't kill you

I most often do not take rain pants. I normally take quick drying stuff. I will take them in winter if digging around in snow. I don't take them on day hikes even in winter (I do take adequate warm clothing and emergency gear including a silnylon tarp/poncho

being wet doesn't kill you. being cold does. its a judgement call but if your friend would prefer to carry a warm heavy jacket and have a cold wet butt and legs let him hike his own hike. if you genuinely feel he is in danger talk it out but at that point you might not want to go if you are not comfortable that he has enough gear to be safe