Try getting your friend to overhead squat with a very light bar or his hands up. When he gets more comfortable getting lower, then go back to the squats. I find a lot of people fear going low on squats because they're afraid they will fall on the butts.
You might also want to try putting a little 2 1/2 or 5lb plate just under his heels only and that should allow him to get lower and more comfortable. If you want to really learn some good advice, buy the book Starting Strength. It's available at elitefts.com and it helped me squat lower and add 50lbs to my max squat in 2 weeks with the info there.
I'm a trainer in my gym and now i have an easier time getting all my clients to parallel squat after reading the book. Good luck.