Watusi are very good eating...even a large bull. It is excellent beef.
--It's perfect [and, IMO, happens to be the one rifle to have for anything, big or small, on planet earth if one is limited to a single rifle and has about three different bullets to choose from depending on the quarry].
I'd recommend a heavy (300 gr), blunt soft-nosed bullet, broadside immediately behind the shoulder/leg and about a third of the way up...as for most larger North American animals.
A quartering shot would need to be carefully placed so as not to blow up meat with grenading shoulder bone; it's too tasty to waste if at all possible. A brain shot from any angle (if it can be executed with absolute confidence) is fine if you don't want the skull for a trophy.
I took a fairly large one a few years ago, with a .458 air rifle...shooting a 500-grain pellet at a mv of about 750 fps. With such a low velocity, the damage is largely just a half-inch hole...but punched all the way through the chest (and vitals) broadside, just behind the shoulder. With that weapon, the kill is by internal bleed-out, so not as dramatic and rapid as will be with your .375 H&H. [I'll attach a couple images; in one the Watusi skull is next to that of an honest-100" Asian buffalo from Australia.]
What a nice invitation for you! Enjoy!