A couple of things to consider.
I think he is going to need a release clip on the harness on at least one of the upper arms. Getting out of continuous webbing is going to be difficult if not impossible.
The crotch strap is going to be vital to keeping the rig in place. It's going to have to be extra long unless the waist strap takes the big detour under the belly.
The larger question is whether this person is fit enough to dive. Can he climb a ladder, do a surface swim, handle a current? Has he honestly filled out the standard medical questionnaire and have any necessary medical clearances.
I guess I'm assuming that most of this weight is fat. If your colleague is a World's Strongest Man finalist, some of this will change.