Short answer, depends on what he wants. If he wants the full red reaver as an animal companion (or just regular companion), then no, that's probably going to be a terrible idea. I don't know what level they are but a free CR 13 creature is usually a bad idea.
If they really want the full thing that's what leadership is for, but I can't even begin to address what level the red reaver cohort would be. Certainly not less than 13.
If they want an animal companion that looks and sort of acts like a red reaver, that's absolutely easy. Find something similar (Megatherium, Ape possibly) and figure out what abilities he wants added (and what he's willing to give up for it). Trade out scent for darkvision, lose 4 dex to get rend, lose another 4 dex to get that pseudo-flight. Remember, -2 dex for +1 NA is basically a straight tradeoff. Get something close to what his idea of a red reaver is and then just follow the animal companion rules.
