Friend sending gym selfies to girlfriend...

[Interstellar]

You’re 80 percent sure? so that means it’s not 100 percent, even if it’s 99 percent you’re still not at 100 percent.

 

[Acacia98]

OP, I think you have two options:

1. Have an honest conversation with her about it. After all, technically speaking, she hasn't done anything wrong. You're just wondering whether your mutual friend has crossed a line.

2. Quietly observe her over time to see if her behavior changes in ways that worry you. 

Personally, I'd opt for the second one because with the first one, depending on how the conversation goes, one could come across as a jealous, controlling person.