My first advice is simple:
Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it. Do it!
You can scarcely imagine how much you are likely to learn by playing with other people. I would (and did) jump at the chance.
Learn songs you like that aren't too hard at first. Get some chord charts. He can just strum to start with, but you'll need to experiment with different rhythm strategies. Maybe the first one will be "repeat the chords in the RH on the quarter note beats, play the root in the LH on 1 and the Fifth in the LH on 3"
Try varying the RH chord inversion so the top note doesn't jump around too much. i.e. follow a C Major in root position (CEG) with a G Major in 2nd inversion (BDG). This keeps the top note constant, which we tend to like.
Listen to various piano chord rhythms and try to work them out by yourself. Then you can try them out with your friend. Experimenting with someone else gives you more focus and more incentive. It's also fun. See if there's anyone else around on your level. Maybe a bass player or a singer.
Enjoy. Don't worry about not knowing where to start. You'll get past that stumbling block quickly, I predict.
