As others have said, if another teenager under the age of 18 sends him a sexual image, and he forwards it on to a friend, maybe to show off, or impress them, or for some other reason, he will have committed a crime. If it appears he or a friend coerced her into taking the image, it will be worse.
In this sort of situation, if the friend forwarded the image to someone else, your son could theoretically be viewed as complicit under "joint enterprise". He doesn't even need to have said anything. Young people have been convicted via joint enterprise because they were simply present at the scene of a crime.
You need to talk to him about peer pressure, and how he gets out of situations like this, because otherwise the consequences for him could be really negative. What if, next time, his friend gets into a fight, and someone gets seriously hurt? Just standing nearby could make him legally complicit. What if the friend sexually assaults someone, and he's viewed as having encouraged him due to messages sent?
If it were my son, unless I was prepared for him to be convicted of a crime, I wouldn't involve the police- you don't know exactly what the other boy has done/will do going forwards, and what might initially be a friendly chat could later be used against him.
At the time, incidents like this are devastating for the girls involved, but ultimately, it's likely she will recover and move on from the situation. But actually, if a crime was deemed to be committed, it would be your son facing really long term, life changing consequences. I think you need to stop looking at this as a favour to the girl/her mum, and start thinking about it in terms of protecting your own son from himself- that might make you a bit firmer, here.
If it were me, I'd really want to know what was on the chat, and whether he was minimising. If you check his phone regularly, he may well delete chats like this as a matter of course.
(This is not me trying to minimise what he's done, btw. But in my experience, a lot of parents of boys think that this is really a big deal for the girl and don't take it seriously. They need to understand that actually, they need to be worried about the potential consequences for their sons just as much, if not more.)