To TS:
Just let her hang out with the guy, and you should be hanging out with other women as well (as friends of course). Then you both can talk about your activities with other people rather than only she talking about the guy. If you dare, you can even suggest that you're both allowed to go on casual dates and flirt with other people, but stay committed to the relationship.

good idea

Its power dynamics. If she feels like she doesn't have to fight for you, she'll easily leave you. You're less valuable. However if you're also out with other girls, that you also have options, she's more likely to stick with you. The idea of allowing your partner to flirt with others in a LDR is to let them get rid of frustration in a controlled manner. It doesn't always work, but at least when you lose her, you'll also have been out with other women, so you don't have to be so damn depressed.