You know, I'm a big fan of doing your own decompression calculation -- I've been diving in gauge mode for about nine years. But I sat back and thought about what this guy might be doing as a DM. If he's just assisting with classes, staying within no-deco limits is pretty trivial, since the dives are shallow and pretty short. But if he intends to do guiding, that's a different kettle of fish. Guided dives aren't always shallow and aren't always short and involve multiple divers whose experience and skills are unknown and sometimes dubious. We all have finite situational awareness, and in the case of someone guiding, my guess is that they are taxed with the requirements of navigation, keeping a group together and monitoring everybody's position, comfort and gas, and trying to find fun stuff to point out to the clients. Although a guide ought to have a good visceral sense of profile, using a computer in that situation is far from reprehensible.
So what computer? It really depends, as always, on budget, and a little bit also on the functionalities you NEED. As an older diver, I NEED a display I can easily read. OLED screens, especially where you can vary the size of the numbers, are well worth the price premium to me, especially since I dive primarily in low light or at night. For a young guide in the tropics, who rarely gets into dark water, that might not be high on the priority list at all. I would think that, with the increasing popularity of "geezer gas", having a Nitrox-capable computer would be a good thing, and it might be nice to have one where you can adjust the conservativism, so that if you are guiding an entire group of Suunto users, you can match their readouts
. Having a user-replaceable battery would be very important to a working guide, as well as having a unit with a low incidence of reported failures.
From everything I have read, the Petrel is a fantastic computer that meets all of the above criteria within the OP's price point. It also has the possibility of moving into technical diving with the owner, who has indicated some interest in doing that. I think the resistance to computer use in technical diving is waning, with the proliferation of units with technical capability -- but I think that is exactly where a user needs to know precisely what the algorithm is that the device has, what the assumptions underlying it are, and what is known (and what is not known but repeated as gospel) about decompression from deeper and longer dives.
