I hope these posts have reassured you. May I add my two-pennorth.
My sibling, who I am going to refer to as X and avoid identifying detail because I am afraid of X: had all the negative personality traits you've described and also alienated from family and, from grapevine information, friends that didn't tow the line. X was also a thief and violent.
My poor mum couldn't take any more of the chaos X created but wanted to know her child was alive, well and functioning.
So, although we knew X was ex-directory we regularly rang directory enquiries for X phone number. Declining to give the phone number confirmed X was at the address, so we knew X was alive and paying the phone bill.
Maybe our experience was more severe than your situation; but I would have been afraid of reprisals had we reported X to any outside agency.
I have very conflicting feelings about my sibling. I wish so many things were different. Sometime after my mum passed, I tried directory enquiries and was told X was no longer listed at the address. I hope X is ok.
I'm guessing, as this site is for Grans, your friend's son is a grown man. I'm also guessing that if she thought so well of you to ask you to check on him she wouldn't want it to cause you any angst. I wonder... If she was alive now would she want to protect you from unnecessary angst.
Bless you for your kindness to, at the very least, a difficult person, but please don't feel an obligation to keep someone hurtful in your life.