Just because she has a friend with benefits (intimate friend), doesn't mean its okay for her to do whatever she wants. Open relationships need just as much negotiating as poly ones. The difference between poly and open is the sex component. Casual sex is more what open relationships are about than poly. Swinging isn't necessarily part of that.
Coming home at whatever hour when that is something she doesn't normally do would be alarming the first time and after that just down right inconsiderate if you've at least asked her to let you know she's okay. I think she might just be stringing you along. It might be that its time she get a job and support her self. At least then you would feel better about making decisions that are based on what is good for you. In the mean time, time to start talking about boundaries I think!
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