Dear John,  

I had a friend too that I used to think should be a priest, not me, and things did not turn out quite the way I thought. Maybe it will be different for you and your friend, so here are some suggestions (a lot depends, of course, on how close you are as friends and how frank you can be with him).

1. Pray for him. Pray that both you and he will be generous enough to do whatever it is God wants of each one. This thought that has come into your mind is God's hint for you to give serious consideration to what he wants you to do with your life. You know that the Church is Christ's Mystical Body, the health of each member affects all the others, and here is one of its concrete applications: you don't make others holy by telling them to be holy, but first of all by striving to be holy yourself. Same if you want to help others with their vocation.
2. Mention it to him. Don't just think it, say it. You may well be the instrument that God needs to make him realize what is happening in his life.

3. Stick together and help each other. Create a circle of good friends, and as well as having fun together and keeping each other out of trouble, do some constructive things that will help others in some way (charitable works, help the poor, teach younger kids the faith, help out some old folks now that Summer is here and the grass is growing, help younger kids at Summer camps...).

4. Invite him on a retreat with you so that you both can pray about your vocations.
I hope some of this helps, and I will say a prayer for both of you.