More to the point: this one depends SO MUCH on how they already handle finances between them. And while I think there's very few situations in which framing it in terms of him getting "a cut" of her profits would be reasonable, I do think if she's moved in with him to the point where she's renting out her own place it's not unreasonable for him to say, "So, we're de facto living together. Are you open to paying me rent?" At the very least, if she's at his place almost every night, I would consider it polite for her to offer to chip in for utilities.