Thanks mrsm, I thought I'd explained but I understand it wasn't clear. Yes I do struggle with this kind of thing. But to be fair, some posters have replied saying they don't see the problem with DS inviting the new friend, so it's not clear to every adult either.
It's definitely not the case that DS fancies this girl and wants to ask her out but doesn't know how to. He does not want to start a new one to one relationship at the moment at all. That's why he doesn't want to ask her for a coffee or anything else that seems a bit date-like.
As to arranging another activity with different friends, that would be tricky as he doesn't have another group of friends outside the sport activity, just individual friends who don't hang out together as a group. So, for instance, he could ask a single friend who he goes sailing with, and this girl, if they want to do something, but that might be quite awkward. I can see it might be a problem that DS doesn't have other big groups of friends that he spends time with, but that's the situation at the moment so there's not much I can do about it. I have been encouraging him to branch out with friendships but he does find it hard to take the initiative. That was why I was pleased he wanted to ask this girl along, but I do understand it's not the best idea now.
I really do appreciate people's thoughts and suggestions. I'm still a bit bemused as to how I should 'back off', though... I didn't ask DS to involve me in this, but I do always try to support him when he comes to me for advice. I didn't interfere in his relationship up to now and I wouldn't want to. I feel comfortable with the amount of support he asks for and I have faith he will gain more confidence to make his own decisions as he gets older and gets more experience. He wasn't ready for a girlfriend before now, whereas I am sure lots of others his age have been dating for years. He may well be a bit of a slow starter but that doesn't mean I need to push him away, I don't think.
"Let him have some fun and stay out of his love life"
This is the kind of comment I don't understand? I've no interest in being involved in his love life except when he asks me to be. But when he does ask me, I'm not going to say, "ugh, cringe, don't involve me." And I don't get why asking MN for opinions is 'massive over investment', but I understand people have their own issues and like to get quite vocal on here. Thanks to those who have not felt the need to slate my parenting.
"She might at a latter stage become his girlfriend or she could just remain a friend or even the girlfriend of someone else in the group. He can’t stop interatacting with girls as he doesn’t want to hurt the ex’s feelings nor would it be kind to exclude the ex so he can met up with the others in the group whilst he gets to know the new girl."
Well, that was how I saw it, too, but I think given ex's vulnerability it's best to err on the side of caution. I'll suggest DS trusts his gut instinct on it (he was concerned about ex, which is why he asked me). I'm just still slightly worried that he'll end up tiptoeing around ex and not feeling free to pursue friendships or relationships that would be positive for him, but it's early days yet.