My brother attended a free seminar hosted by a company that specializing in the topic that was presented. Part of the sign up form for attending the seminar stipulated that he would not be allowed to work for a company who also focused on that subject matter for a period of 4 years after the seminar. He now wants to work for a company that focuses on that subject matter and has only just realized that this is an issue.

Is a stipulation like that enforceable? Would it hold up in court? Could he potentially get sued?

Just a few other details that may be important. My brother lives here in Toronto with me, but the company is based in the States and it's been just under a year since he's taken that seminar.

Thanks in advance.

Yes, non-compete agreements can be enforced. But since he did not work for the other company, he just went to a meeting.

Now here is the issue, to find out if they can or can't enforce it, means that if they sue, he will have to fight this in court
He may win, he may loose, but in the end he has to be ready to go to court if they find out and decide to do something