It seems the guest thought should was booking an entire home and would have the place to herself, thus not having to disclose that she would be bringing her partner without telling you. Or maybe she called on her boyfriend to join her after realising that she didn't want to be there alone, and didn't intend to have him stay originally. Either way, she should have informed you, but did she actually stay or not? You say you refunded her, so did she leave straight away or did they actually stay one night in your apartment? If they left without staying, you don't really have any recourse, as you authorised the refund, and they guest didn't stay in your property. I think to call them scumbags is a tad harsh, as there's obviously been a bit of confusion. I'd just put it down to experience and move on.
