Oh man! I can understand your concern… To many of us with multiple buns, bunny humping is just something we’re used to, but it can be a shock if you’re not expecting it!
As Jersey and Sarita said, both boys and girls can hump. Two boys can hump each other, two girls can hump each other, and obviously opposite sex pairs will hump. They do this bc of hormones, or to show who is in charge (dominance), or just bc they are excited and/or want to play.
It sounds like you are assuming that the bun who humped is male, but that bunny could still be a female. The best thing at this point would be to get them both in to the vet for a “new bunny exam”. The vet will confirm their sexes, check them over to make sure they are both healthy, do a fecal test to check for parasites (you may have to request this), and discuss proper diet and care with you.
The vet you called is correct in that boys are usually able to be neutered at 4 months, and girls at 6 months. So he will also go over that info with you at the vet visit…