I've experience this before as well. It happened a couple months ago actually, an old friend of the person I'm seeing said some things to me about how "She will never settle for you, she's seeing others and leading you and them on for attention." Some very harsh things that are relationship ending if true, which she knew. I asked her for proof and she failed to follow through and said "just telling you what I know", and that's what I hope your girlfriend is doing and is trusting of your word over this other person's. It turned out the toxic person felt entitled to me because they had met me first, and were jealous of the happiness that we had. I was introduced to my girlfriend through her and she has apparently said mean things for years and I never knew. It caused a lot of drama at the time over a long-distance relationship we had just reconciled, and she knew it would. You've found out what you need to know about this person Stella, best you can do is hope your girlfriend trusts you over this other person first and hears what you have to say.