OP, you said upthread: don’t think I can bring it up without causing a lot of trouble.
Remember, the only person who’s caused trouble is your DP. Possibly your friend’s DP, if her suspicions are correct. And the other members of the group chat who didn’t challenge your DP’s messages.
I don’t agree with looking at people’s private stuff, including phones, papers etc. It’s a breach of trust. But, once your friend had and saw what she saw, she was a good friend to tell you. Once she’d told you, you had to speak to your DP about it.
But, kindly, IMHO you should have held off and warned your friend you were going to have to say something. And ideally your other friends whose DPs are in the group. You’ve moved so quickly you’ve blindsided them all, especially the friend who exposed the group chat, who now has to lie to her DP or come clean. It’s understandable. You were shocked and emotional.
If she’s usually a good and true friend to you, try to meet her in person this weekend. Thank her for letting you know. Apologise to her for telling your DP about the group chat when she’d asked you not to. Acknowledge how much stress this has now caused her too. But ask her to understand the stress her snooping and revelations put you under. Acknowledge that you reacted too quickly, without considering her. Offer to continue to support her through her relationship problems. Ask her to support you.
It sounds like there’s some horrible sexist, puerile and disrespectful behaviour in that group chat, and you and your friend need to stick together in dealing with it.
The only way your DP, and the entire group, can come back from this is to hold their hands up, take accountability for their behaviour (including other men in the group not challenging it), apologise to all concerned and explain what they’ve learned to take forward.
If he/they don’t. But instead (as appears) focus on the deflection of who ‘grassed’. The rationale being such messages are OK as long as they’re kept within the virtual ‘locker room’, ‘boys on tour’, ‘lads night out’, ‘bros before hoes’ male circle. And the only problem they see is if there’s a weak link in the chain. If that’s what he/they do: double down and look for someone to blame other than themselves. Then, all you women have your answer.
Hold your heads high. Set your standards high. There are plenty of much more attractive fish in the sea.