The other girl is sam's mother, Amber. To get that scene, you have to reject aiden's first proposal for a task at the back alley then send Dylan to a boarding school along with Alyssa's son, Zac. The pool event it's sort of goodbye before leaving for the boarding school. Now, is it worth it? Well, that pretty much depends on you and the path you're playing and what do you want to get. For some in certain paths, it's worthless, for others...it may be an interesting alternate timeline or current events and for those in just Dylan's path...it's a bit unnecessary, but at least it's fun to watch.
We don't know for sure what will it become of sending Dylan to the boarding school compared to let him stay and have the bikini fashion show just with him, instead of having the event along with sam, but IF you send him to the boarding school and have the pool event with Sophia & Amber, there are some faint hints of something happening between Dylan & Amber that could continue in the future, leaving to wonder what else he would do with Amber and If Sophia finds that out, how will Sophia react to it?. We don't know for certain, maybe something does happen or maybe not.