I'll speak from the experience side of owning a Mbu. From the behavior that I have seen, young mbu's are very curious and love to "test" everything. The issue is that when they are "testing" things around them, they have that sharp beak that shears through whatever they are testing. My fear would be the eyes of the ray would seem mighty interesting, and may be chomped. With this being said, I have not actually tried to have my Mbu with any rays, so I cannot say that with 100% confidence. I have had my Mbu with polys and a few other fish that he did nip at out of curiosity, I ended up having to remove them. He is now riding solo, lol.
I am glad you posted this Kevin. I was going to have Mike order an Mbu for me and originally decided against it because I was afraid he wouldn't get enough to eat due to competition. I plan on getting a ray or 2 in the next year, and this would have been an additional potential problem I could have had to deal with.