He will be fine. He might not still have his job in a few months but other that, he will be fine. 300 CPM isn't even close to dangerous. You get a higher dose every time you fly in an airplane, or go to La Paz, Bolivia.
Yes, but: depends on what he ingested. Deuterium/tritium cannot emit Alpha particles, the ones blocked by your outer/dead skin (also they're an order of magnitude more dangerous than the other at equivalent grey), so he's fine in that way (unless there is more in the pool, but that would be a bigger issue).
The issue with ingesting deuterium/tritium is that the dose will now have to be calculated/estimated per organ, and while I don't remember exactly how it's done, it's more complex than calculating the mSv he took (I can't give you more details, I'm not competent, I've observed a radiologist a week my bachelor year to decide which master I would do, I'm now working in software which should tell you everything)
