So I was trying to figure out how to broach the subject with my neighbor (I had yet to go over there since I started this thread), and an unrelated pretext presented itself today where I could go have a conversation with him. Two other neighbors have had the same plumbing failure, so I wanted to share that with him so that he could hopefully avoid it. So I do, and after that chat I just nonchalantly say "How's the pool doing?"
He admitted that it had gotten away from him, so I offered again to help him with it. He reminded me, again, that he's had pools before, as if to say "I know what I'm doing," but then he did say that maybe he'd take me up on the offer. He said he just hasn't had time to deal with it. And we all know how that can be. So I say "Any time, glad to help," and resisted the temptation to go into why he needs to take care of it sooner than later.
I think he would rather I fix his plumbing than his pool, but plumbing sucks so I didn't offer that. Anywho, did what I could. And while I doubt he'll let me help solve his pool problems, at least I can feel a little less of a jerk about it...
Thanks again all for the perspective.