Sounds like Hasselblad's mechanical vignetting issue on their earlier bodies(500C,500CM,503CX,CXi, 500elm/elx)
Hasselblad redesigned their later(mid/late 80's and beyond) bodies(501CM,503CW,553ELX,55ELD, etc.) to accommodate this issue. They put a bigger mirror into those later-model bodies. I wasn't aware that Rollei didn't rectify this seeing that Hasselblad did in fact change, but it seems(according to your post) that this sounds like it's the case(unless your friend has a 501CM, 503CW, etc.(a model with that larger mirror))
Anyhow, shoot away. It's not going to affect the film itself, just what you see in the viewfinder. Of course if you compose right out into the corners to maximize frame space, then you might be buggered.
-Dan