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.

[MsJayne]

She knew him before she met you, if they were going to have some sort of interlude they could have done it then. I think it's more likely that he's just a dude who thinks he's pretty hot stuff and probably sends his gym pics to every person he knows because he thinks other people want to admire him as much as he admires himself. You could get a lot of hilarity out of this if you look at it the right way, make a joke out of it with your girlfriend.