1-test isn't a prohormone, it's actually a steroid hormone itself. The problem is, it doesn't have too great of an oral bioavailability, and it hasn't been made into a reasonably painless injectable yet.
I wouldn't really recommend it because most guys say they get shut down pretty hard on 1-test and have a great loss of libido. I wouldn't want this effect combined with the same effect from deca. But then again, 1-test can cause joint pains and deca would probably alleviate that.
Personally as a first cycle I wouldn't recommend doing 1-test if he's already doing deca.