This is definitely a very platonic and girl friend thing to do. It really is just that: hyping each other up. Men do have a hard time understanding the concept, but it really is just a really great feeling to be complimented and hyped up by a platonic friend who you know has no sexual attraction to you. It’s ~kinda~ the same concept as a mother’s compliments....People say a mother’s boasts about you aren’t valid cause she’s your mom. Some women feel that their partners compliments aren’t so genuine cause their partner is having sex with or wants to have sex with them. A girl knows her friend probably has no ulterior motives (like wanting to tap that) and is therefore trustworthy in her opinion of her body. Plus...girls notice things guys don’t? Like...idk her friend may compliment specific things about her pics that you are just “guy blind” to. 

As for being mad...I mean, would you want her to get mad at you for being in, say, a locker room naked with your buddies? For smacking each other’s asses after some athletic thing? For some other bro-like thing that also happens to include physical aspects? Wouldn’t you be more upset that she’s assuming just cause you two dudes saw each other naked that you must have some underlying sexual tension/attraction? 

You do not own her body, and just because she chooses to show it to her platonic gal pal, who has the same anatomy, does not mean she’s betraying you. No offense, but your jealousy over her relationship with a friend is your problem, not hers. Alienating or forcing her to adjust her friendship dynamics may be unfair and cause undue pressure and tension on your own relationship with her.

Then again, if she is actually lesbian or bisexual or secretly fulfilling a sexual urge with her friend, then you are valid. Just sit down, without judgement on both sides, and discuss it and what it really means to her, with her.