I've posted a few times here, particularly following my breakup with my non diagnosed ex. In a relationship for 2.5 years, he wanted me to move in, house a mess, he didn't work, lack of empathy Many of the toxic behaviours came out, however like so many he could be very likeable, charming and he loved sex and loved showing me physical attention, though it was increasingly apparent that if something was distracting him I would literally be forgotten. 
I knew he had a crush on one of my friends (who is pretty and has a similar sense of humour to me and my ex, she is engaged and has a toddler, her partner has a very good demanding job and he's great at diy)

We  split in September for 2 weeks as I believed he was focusing too much attention on her during the course of an evening, it was insulting. Looking back I could see she appeared to enjoy the banter/flirting whilst her partner was tending to their little girl more who was being a bit demanding and I was aware my ex and friend had probably about 10 minutes when they were alone walking back to the campsite at night, as I was seething and charged ahead.

Anyway fast forward our proper split in January, and my friends and family have been very good at highlighting all his bad points and how he'll take his problems in to the next relationship he goes to. Apart from this specific friend, very measured and no comment when I expressed concern he'd move on quickly, due to the adhd. 
With social media it's sadly a reality that you'll often look and the only link I had left with my ex a few weeks after my split was WhatsApp. I looked one day and noticed he was online,I was then seeing my friend later so switched to her and noticed she was online too. I did immediately have a funny feeling, and so I asked her if he has ever reached out, she immediately replied no, asked why. I explained he has never made me feel fully secure with regards to other women. She said she would tell me if any partner ever reached out. 
Mind was put at rest for a few weeks, to the point I was going to block him, now I have an anxious attachment type and I thought I'll check WhatsApp to prove I was imagining and reading too much into my ex and friend. Though I saw it again, and I became consumed for 2 weeks looking, and their overlaps were ridiculous, I compared it to over contacts, which maybe has 1 or 2 at most overlaps with the same person but never continually with the same person, my ex and friend has up to 9 a day. Twice when I saw they were online at same time I text her at same time and she immediately went offline. 
I asked to speak to her for a chat, she said it sounded ominous, when we spoke I skirted around the topic just talking about the struggles I'd had with my ex and the breakup, and she said she thought I was going to ask if i was going to ask if she'd been contacting him, that would have made her sad, and she admits it quite upset her (even though all is said was was she free for a chat). The overlap seemed to stop for a day or so then picked up pace again. I couldn't ignore it so I calmly asked her that I was aware of something and so categorically has my ex been contacting her. She said no and said she was on WhatsApp mainly morning and evening (this wasn't what I noticed, mainly afternoon, when she was at work) she said she'd tell me, and that she didn't think she was his type anyway. 
The next day she said she wasn't joining her fiancé and child to go to Portugal for Easter, and was looking forward to a break, I thought that was a bit strange and she also said she'd once had an affair with a married man who she says they used to laugh a lot and he periodically reaches out to her, sounded like she hadn't replied saying leaving me alone I'm engaged and she said her partner wasn't aware, but if he saw the messages he would see she wasn't egging him on. 
That really unsettled me, so I said the WhatsApp similarities I couldn't get my head round, she said immediately I should look at our other friend I said I had and others, and it wasn't in the same level of frequency and in the afternoon. She immediately  said I was mad and that's the end of friendship and that she wasn't even interested in my ex and she was engaged (I noticed she never said she loved her partner, and in recent weeks has said he's her rock and has always been there for her - that's not saying she loves him though, she also hasn't seemed excited that they are looking for a new house). 

I thought the next day I'd look One more time to see if there was any change in their WhatsApp.... 
Absolutely no overlap at all, closest being 20 minutes. Gone from daily multiple  overlaps for 2 weeks , to zilch. 

I've gone from processing a breakup, to now being in turmoil about deceit and lies and my gut instinct and you question so many things about loyalty and a friendship. I get that my ex is a free agent, what do people think? Smells of thrill seeking reckless behaviour.
Surely if something was happening, he'll just revert to the same adhd negative behaviours with her too?