Let's get this out there right away: no matter your talent level or level of experience the number of keeper frames you can generate per day will be lower shooting medium format for wildlife vs 35mm.
You will miss more shots. You will be unable to take some shots that you otherwise would have found easy.
The gear is very unforgiving and demanding on the photographer; no IS, larger mirror, much slower AF (or no AF), slower frame rates etc
That said, I've seen some incredible wildlife/bird work shot on MF.
It's not normally what I'd suggest (and my job is at a dealership that sells Phase One and Leaf!), but that doesn't mean you can't do it. Sometimes it's fun to use the "wrong" tool for a job.
So your friend should think about what his goals are. If he will cringe because he missed a cool shot then this is probably a bad idea. If he's okay getting less shots, but (potentially) loving the handful he does get then it's at least worth some testing/evaluation.
I would really really really suggest he rent or demo before he commits to *anything* in medium format. Most pro dealers can arrange a rental-towards-purchase (at least that's true of us).