I was the child in this situation many years ago.
When I was training at a national level there was a 30 year old (ish) man that swam with us twice a week. He was training for a British record and the masters sessions, which I think may have been about 5 hours a week, just weren’t enough.
The head coach agreed he was able to swim with top squad as long as he had a DBS check done, which he did and obviously passed!
Honestly, if this person is training to the same level & keeping up with your child, who I can only assume is at a very high level, then I highly doubt there is anything more to it that wanting to achieve a specific goal.
In regards to accidentally getting injured etc, is there that much difference between being accidentally hit by a athletic 17 year old boy and a 40 year old? I suspect they’d probably do roughly the same level of damage!
And I doubt the coach is giving that much coaching time to an adult - at that level nobody who is swimming should need too much guidance.
Ultimately, if you’re that bothered by it then you should look for another club. From personal experience though, I can say it was fine - but that’s just my opinion.