On his way home through London on Saturday, my DH had a bit of an altercation with a gang of oiks, ring led by a very mouthy & not very bright short @rsed teen, who pretty mucvh threatened to rob him 
Now I say not very bright, because even though he's no longer a "spring chicken" so to speak, I don't think many sane kids would threaten to rob a 6'5" plus leather clad hairy @rsed biker & not expect trouble back 
The situation was diffused by Oiks mate suddenly piping up "hey aren't you DDs Dad, XXXX Husband, My Mums old friend.....to which oik looked shocked & piped up "what, you know him"
....DH then got into a chat with ex friends Son, asking where they were off too & finishing off with lovely to see you, but please make sure you look after yourself
DH came home genuinely worried for the lad as he's never been a trouble maker, did well at School, always been a smashing Kid etc, & is obviously flexing his now adult (18) wings, but as DH said, he is mixed race & with a mate that behaves like that & shows no common sense, then London been what parts of it now are, its only a matter of time before he gets into trouble, God forbid nothing worse.
Now sadly I fell out with his Mum recently & her own behaviour (IMO) has been quite odd, so not sure she would appreciate my contacting her over this, but if it were my DD I think I would want to know....the Lad does still live at home, & I know his Mum would be mortified, & I suspect half of her recent odd behaviour is down to the Kids growing into adults& her losing control & struggling with that, she can be a bit of a control freak with them
But DH WAS genuinely worried & I've known this lad since he was born AIBU in wanting his Mum to know, & if so how to go about it without causing more resentment 