Answer:
It is not going to cause an infection. Don't worry about the pool water. Water doesn't go into the vagina normally unless it is forced in. Even if water did get in there, the tampon would absorb it. Seriously, not worth stressing.
It depends on how long she had it in before that. If it wasnt in very long before she should be fine, if in was in longer then watch for symptoms of infection. I highly doubt anything will happen! so just relax and wait to see what happens! good luck to your friend!
As long as it wasnt more than 8 hours she should be fine!
She'll be fine... and she can't really hate them too much if she forgot it was even in there! lol