The group chat

Reacting tells Dev the group approves, and makes the next share more likely. You did not post it, but you have joined the audience she never agreed to.

You are no longer part of it, which is something. But the photo is still there and nobody has objected. If speaking up in the group feels like too much, a private message to Dev does the same job.

Now you have shared it too. Sharing someone's intimate image without their consent is a criminal offence, and "someone sent it to me first" is not a defence.

You have named the problem in one line and said what should happen next. One person objecting is often all it takes for others to agree. It also tells Dev, as a friend, that he is putting himself at risk.