How about gently talking him through what you are doing when you next go out together and you are P1.
Also tell him why you believe it is not a good idea to swap tanks just before take off but try to be chatty about it.
If he has got anything about him he'll pick up on the good ideas and adopt them himself.
We have all got too low when flying into a strange field at some time but do I suspect that he didn't do enough to correct things and got behind the aircraft?
It is up to you to decide whether you can improve your friend's flying by gentle encouragement and keep flying with him but you must make your mind up now before Darwin does it for you.
Just had a final thought why don't you suggest that you both sign up for some coaching "to improve our flying"... I believe the PFA offer this service,
B