Well, what does your friend want to play?

Tanking with a Warr would be an option, as this requires some action. So you would go for 3 DPS and 1 Healer - should be fine.

Or ... your friends plays the healer (Priest) with you playing a prot pala for tanking and 3 DPS. Fine too. Healing is more boring than tanking imho - if you can handle healing and DPS then probably the Warr would be more fun to play for your friend as he will always be in the "center of action".

I would go:

Warr -> Friend
3 x Locks for DPS and some CC and 1 Healer -> You