Being a DI shouldn't be a problem, but that does mean leaving for sea a few weeks later than everyone else. That may mean staying out longer during the second sea period and having to do more training trips and/or watches when back at school.
Sailing teams are in season in both the fall and spring, exactly when B-splitters are at sea. Hate to burst your bubble, but that pretty much makes it impossible to compete with the team if you're a B-splitter.
The good news is that since there's still plenty of time until next summer, you can switch to A-split. In fact, participation on a sports team (especially waterfront teams) is a big factor on which sea split you get. I would have your kid look into this. Do it now, since there are a few differences in courses to take during the third tri between A and B splitters. I don't remember exactly, but I would think the Dean's office is in charge of this.