It looks like she wasn't with the group -- she was already there, and the boy ran into her (he knew her because she was a neighbor). (That's from the latest story, that ragman linked to.)
It's coming together as a plausible but unfortunate tale, something like:
- The pool has been murky for a while
- The boy ran into an acquaintance (Joseph) at the pool
- He saw her go down after the slide into the pool
- He got a lifeguard's attention, this one said he/she was on break. (Jerk.)
- He got another lifeguard's attention, this one said that he/she would do a check of the pool in a minute.
- The boy had told authorities and they said they were going to do something about it -- he went back to what he had been doing (playing with friends/ relatives.)
- If this is how things went down, this is where he should have been a lot more insistent about making sure that the lifeguard actually did what he/she said she would do -- but the kid's 9, and I get how it's possible that it's not how things happened. I'd suspect that around here he was feeling guilty and trying to put the whole thing out of his mind, too.
- No search took place.
- The kid and his party (which had not included Joseph) left.
- The kid says something to his mom but she doesn't think he's serious.
- The kid has told authority figures and he's a guilty, confused kid.
- Joseph stayed at the bottom of the pool for a couple of days, unseen because of the murk.
- Joseph's body started decomposing, and gases etc. caused it to surface after a couple of days.
- The first people to see it were the after-hours couple. (Good thing too, that it was them and not a passel of little kids arriving in the morning.)
- Kid is extremely upset and blaming himself for not saving her.
If this is actually how it happened, I think the lifeguard who said he/she would do a check, and didn't, is the chief villain. With the pool management who decided to leave the pool open when it was illegally murky in close second place.