“Hanging out” outside working hours isn’t work. Anything done without the meter running isn’t work. That’s when you being to veer into slippery slope territory. So yeah it’s probably a good idea the client be aware she has an SO.
I agreed 100% for a moment, then changed my mind. I'd allow the SP the right to decide whether it's work.
She could be doing it for fun (e.g. attend a restaurant she wouldn't otherwise). She might do it for a friend, if she considers the customer a friend.
But she also can be doing it as a bonus for a paying customer. Regular customers can be offered bonuses in different forms: cash discount / extra time / extra services. Some social time with customers does not sound too dangerous. It's about as normal as it is in other industries.
Of course, there is a slippery slope if the customer sees it as a date. It can be manageable. If there is no intimacy (kisses/hugs) involved, I wouldn't set my hopes high if I were the customer.
It can get difficult if the customer relationship pre-exists her SO relationship. Then you don't know just how "special" the customer relationship used to be. She might find it hard to either disclose that to SO or risk her regular customer. If a customer effectively pays her rent, she'd think twice about changing anything.