I need some help from wise Mumsnetters to help me get some perspective. This is a long one so thanks if you stick with me but I don't want to drip feed.
First a bit of background - DD1 is 19 and was diagnosed with ASD 2 years ago. She has always been involved in lots of extra curricular activities, but felt on the edges of everything and had quite a lot of social anxiety. She relied very heavily on me for emotional support and as a result we were very close.
Last autumn she started a university level course at a local college as she didn't feel ready to move away from home. No problem, we've (DH and I) always taken things at her pace and encouraged independence within her comfort zone.
Since starting the course she's become much more independent - which is great and has helped me to realise just how much I gave emotionally now that she has stopped relying on me so much.
She has a boyfriend who is on the same course and is a few years older than she is. She was upfront when she started going out with him and told us that he smokes weed. We encouraged her to bring him home to meet us, welcomed him, offered food, drink etc but pretty much left them to it. He had his own place for a while and she went there quite a lot, staying over a couple of nights each week.
Now he has moved back in with his DM and DD goes over there 4-5 nights a week and ends up staying over mostly.
He hasn't been to ours for months now and says it because he's scared of me because I'm such a strong woman and he finds me overbearing. I am a strong woman but I've purposely not gotten involved in their relationships because I don't want to be one of 'those' MILs 
. The most I've every spoken to him is to offer drinks or invite him to join us for a meal - basic hospitality really.
The problem now is that DH and I are getting worried that there may be some EA going on and we're not sure if we're handling it well (hence us both agreeing to post this on here).
Lots of little things are being mentioned such as her having to hang around a lot waiting for him whilst some friend or other has a crisis and needs him. He can't be contacted as his phone was stolen so she has to contact him via random mates or family. Apparently he has had a terrible run of bad luck, such as being mugged, beaten up, framed for a crime which has left him owing £1000s etc.
He's promised her loads of times they're going to do stuff or he'll buy her gifts and it never comes off. There's always a reason why not which DD takes a face value.
She says that she puts up with the disappointments because she loves him and when it's good it's great.
To us this is all massive red flag time but we're not sure if we struggle to trust her judgement because of her ASD and lack of maturity or because he's gaslighting.
We're constantly torn between wanting to tell her our concerns, and yet keeping schtum in case this pushes her further away - she's already coming home with plenty of attitude which we do pull her up on.
For now we just keep being supportive and say 'that's a shame' whenever we hear of the latest reason why he's had to let her down but it gets harder each time.
So what would you do or what have you done if you've been in a similar situation either as the parent or the teen?
TIA