Honestly I am surprised that there is no one mentioning that your son is an adult and that he needs to make his own mistakes.
Yes, he may have a disability. But if you believe that he cannot function as an adult with all the rights and responsibilities then you need to get a lawyer and get power of attorney and/or conservatorship over him. That way he won't be able to go and see her.
If he is indeed functioning at a level that does not preclude him from having all the rights and responsibilities of an adult man, then you need to butt out and let him stretch his wings and fly. Just let him know that he can always return to you and that you will always be there to pick him up if needs be. No questions asked or "I told you so's". I wouldn't be surprised if he came back after a few weeks... these kinds of romances tend to burn out fairly fast.
On the other hand I would also make sure that he finances his little endeavor by himself. Responsibility comes with the rights... so no $100 debit card, but maybe a prepaid cell-phone for emergencies.
You are not doing him any favors by keeping him dependent on you. Honestly if I was your son I would be embarrassed and furious about the depth of your involvement as well: having a talk about being safe and legal requirements to work in Canada - fine - stalking his acquaintance online and having relatives call her anonymously - not OK.
Talk about a helicopter mom...
Good luck.
ETA after reading your SWH:
I am sorry my answer offended you. That was not the intention, but I appreciate that it got you thinking.
I work with young adults. I just recently had a few in depth discussions with clients who feel very resentful towards their overprotective parents. Things like mom's getting involved in their relationships, meddling in school or college, even one person whose mom would call the young man's employer to demand a raise for her son! Needless to say the guy lost his job.
The overarching result is young men (mostly, some women) who feel that their parents have socially stunted them and made it harder for them to live adult lives.
A certain degree of trauma as in broken hearts, disappointment, rejection, making mistakes and dealing with the consequences is a necessary, absolutely essential part of growing up. By protecting him from getting hurt you are only keeping him in an immature, gullible, naive state.
I do understand it, really I do. My daughter is little and when she gets her feelings hurt at school my heart breaks for her every single time. But honestly: getting her feelings hurt and her heart broken, building and loosing friendships is how she will learn to judge people's character and eventually identify those people who are compatible with her to for real, long term relationships.
Unfortunately your son seems not to have had that opportunity when he was younger. Maybe it is because of his disability (and I absolutely understand that that makes it much harder for him than say my DD), maybe because you protected him, probably a combination of both. But you have to face the fact that you can not screen his friends, call them, make sure they are good for him forever.
Sometimes the best intentions backfire terribly and I am absolutely sure you are a wonderful mom with the very best intentions for your son.