Either way will work... your friend can either heal OR tank. Really, they should do whatever you are not the best at If you did the healing, that would technically be a little less position dependant (meaning you wouldnt have to move much) so keep that in mind when choosing. I agree with the other poster... you should do 3 locks for your DPS (from what you have already) and maybe the pally as a healer (who can possibly tank in the future).