I couldn't tell from your note if your friend was having a good time or not. Maybe he's not as excited about playing as you may think. As for following the rules, if you're not betting, enjoy the companionship, and drop the advice unless he asks. I do agree he needs to pick up his pace somehow. Sounds like he spends a lot of time with practice swings, etc.?
Assuming you can only devote X hrs. per week to golf, mention to him you've been invited to play with the other guys, you think it can help your game which is one of your goals, and would that work for him? That should start the conversation so each of you can decide what you want out of the golf games you play.