As mentioned, that is a very unwise thing to do.. that salesperson was not thinking at all and had no knowledge of keeping bettas, let alone breeding them.
The 5 girls *may* be fine together.. or you may end up having to separate some into their own tanks at some point - whether be a few days or a few months - as females are just as aggressive as males and don't always like to live together.
Don't add in a male as it's just going to end up dead as the females will attack him and kill him.
Make sure you set the tank up properly for multiple females - as females are aggressive as males (they aren't territorial like a male, which makes it more possible that some can co-exist together).. make sure the tank is heavily planted - want tall and mid sized plants front to back with at least 3 lines of sight broken - [ '.' ] to help females being chased get away. Sometimes you see pictures of people's sororities with little to no decorations/plants.. but that is just a bit risky and for a 13yr old, I would try to get the tank as close to perfect as possible to make keeping a sorority a much easier/smoother experience.
Make sure she is aware that with 5 girls her tank is pretty much stocked, adding any more fish your bettas will start to feel cramped/trapped and will lash out.
It's doable, and I really wish her luck as owning a tank is great - my first ever tank I got when I was 11-12 and I'm thankful for it. Just in the future, take what you hear from Petco/PetSmart employees with a grain of salt.. and then come home and research before committing. Will save a lot of heartache in the future when it comes to keeping fish.
You are welcome to ask any questions about keeping bettas.. we are happy to help :)