Hello all. My 19 year old daughter (has high functioning autism) hasn't had a great deal of life experience, and has struggled socially over the years. She is generally in a better place these days; she is working, and building positive relationships with colleagues (it was a struggle sometimes at her all-girls' school), socialising a little, earning money, and pretty much enjoying life. 
She is a lovely, lovely girl and you would all love her!
During lockdown, when she couldn't work, she was gaming with an online 'friend' she sort of knew from before. This friendship was a lifeline to her during that time. It was nice to hear her guffawing with laughter, and improving her language skills at the same time (he doesn't speak much English, and doesn't live here). They would also chat for hours and seemed to really click. He's a bit older than her - at 26 - but they seem well matched in terms of maturity levels.
Months passed and you can probably guess what happened. They became more romantically involved (or as much as you can when it's online!), and she is now his girlfriend. This is her first experience of a relationship with the opposite sex.
He makes her happy, even though I've been concerned with some of the comments he has made to her. These are quite controlling, such as him not liking her having interactions with the opposite sex, and questioning her about her FB friends. His previous girlfriend apparently cheated on him. It's all quite intense 
She and I have chatted about this. It is so important to me that she talks to me, and that we keep the lines of communication open. It's a fine line between not wanting to appear too judgy, so that she clams up and won't tell me this stuff, and gently making her aware that these could be red flags. She assures me that she is the one in control, and that she doesn't put up with his nonsense. She challenges him on it, he takes it on board and apologises, and then things return to normal. She is wise, bless her, but can also be naive.
A while back, he sent me a FB friend request, after checking with my daughter that it would be ok. I think he wanted to do the whole 'getting to know the mother' thing. FB Messenger is their main means of communication, as well as phone calls, and the gaming. Then after chatting about it with my daughter, he sent me an introductory message, which he was apparently very nervous about. I sent back a polite and friendly reply. I speak his language pretty fluently. Daughter was delighted that we were 'getting on'.
Since then, he has messaged me on average once a week, to say hello and ask how I am. I engage politely and briefly. When I casually tell my daughter 'oh, I heard from X today', she doesn't know about it. The messages have been nothing but innocent, but I'm not sure why he's in touch really, and I don't feel totally comfortable. I'm a single mum, and this could just be me being a bit jaded about men, and seeing red flags where there are none.
My daughter's happiness means everything to me. But I'm worried for her. My daughter told her younger sister that she is going to move to this guy's country to be with him, when she can, and this scares me. I want her to keep talking to me, but I can't be too scathing about him, as then she'll go off on one and refuse to talk.
In some ways it has been really positive for her, having him in her life. She really values her relationship with him, and this seems to be entirely mutual. But several things she has said have had my spidey senses tingling, as he can be slightly creepy and controlling.
I asked her if they have any plans to meet, and we have agreed that if/when they do, it will be in our hometown first.
How would you handle this situation, and what would you do about his messages? I am doing my best, but am aware that she's an adult now. It's a fine line. I just don't want her making a big mistake, and then turning round one day and asking me why I didn't get more involved 
Thanks in advance.