Although I don't think sharing a computer is a practice to be encouraged . . . before we blast someone for doing it, let's look a bit at what he'd learn from the eRDPml. NDLs for a first dive to 60 feet (the recommended max for a new diver) is 55 minutes. This, of course, is assuming a square profile, so all the time is done at 60 feet.
Now assume the diver is carrying the "usual" beginner tank, with about 80 cf of gas in it. And assume he has the "usual" beginning diver gas consumption, in the range of 1 cf/min. At 3 ATA, he's going through 3 cf/min. Assuming he wants to arrive at the surface with 500 psi, he has 70 cf he can use, or about 23 minutes of bottom time. He's SO far from no-deco limits that whether he is precisely at the same depth as his buddy or not just isn't going to make a difference.
If he goes deeper, his bottom time is going to get so short, it's still going to keep him out of trouble from a decompression standpoint (he can, of course, run short on or out of gas!)
It's only when people's gas consumption drops (usually through experience) and/or they begin to dive much larger tanks, that going into deco becomes a significant issue, even on air.