My 16 yr old brother is in a relationship - need advice

Assalamu alaykum, brothers and sisters. My younger brother has been acting different lately and our parents suspected he might be talking to a girl after hearing a female voice on the phone. A few days ago he left his laptop open and they saw inappropriate pictures and asked me to go through the messages since they don’t speak much English. I discovered he’s been seeing her for a few months. They live in different towns and go to different schools. Both of them have been very vulgar and inappropriate in their chats. It hurt to read those things - he’s my little brother and I never imagined he’d be like that. It was really upsetting. We know her name but not much else, and from the messages it seems her family might be strict. She is not Muslim. He’s hanging out with friends who encourage this because he isn’t surrounded by practising Muslims where he is. We live in the West, and I can see he’s drifting away from his deen and also becoming distant from our family. She also appears to be struggling with her mental health and there are messages suggesting she once tried to harm herself, though I can’t confirm it. They’re very dependent on each other, texting and calling constantly, and he’s always distracted. He doesn’t yet know that we found out, so we want to handle this carefully. It would be easy for him to paint our Muslim family as the enemy and cling more to her and his friends, so we want to be wise about how to approach him. We honestly don’t know what to do and feel lost. Please share practical advice and duas. Jazakum Allahu khairan.