No matter how you look at it, this is a form of cheating. Although it is a remnant of her habits as a prostitute, she now has a partner and lives with her. Even if you were single, there's no reason to just flash your vagina or panties to a virtually unknown girl. No matter how some in the comments try to gloss it over, it is simply a betrayal of trust. The protagonist trusted her and in return, she took a girl in need of sex to her house secretly from her and showed her her crotch, for whatever reason. This is how cheating in relationships begins. She already gave clues when she said she felt lonely.