In our neighborhood where pretty much all of my DD's friend's families (including us) have pools, the rule with all the parents and kids is that if someone wants to swim in someone else's pool the parents must speak prior to getting in the pool. During the summer it is not unusual for the kids to have their bathing suits on so jumping in the pool would be real easy, but that is the agreement that we all made. If the kids are coming over to our house, I usually check with the mom's ahead of time so I know who can and who can't - it's all or none when it comes to the pool.
I would nicely chat with the other mom - if this is a new pool, they might just not thought about it being an issue.
I feel a great deal of responsibility when I have other children in my pool. I know DDs abilities, but not always the other children.