I volunteered at a community law center and ended up being being involved with a situation similar to this, you're correct in that there is nothing you can do. Adults without recognised mental health issues are free to make their own decisions, even if they are bad ones.

The sad part about all of this is that J has friends and family who know what is happening, but there isn't a single thing that any of us can do to stop this. :-(

This will sound unpleasant, but the best thing you can do in this situation to not support her until the scam runs its course and she no longer has any capacity to send money to this person.

I have no doubt that they will continue talking and she will continue sending money until she has nothing more to give - and even then she will probably keep on believing.

I've had some contact in dealing with people who are the victims of scams, this statement is not true.

The reason the victim goes along with the scam is that they are receiving emotional validation from the scammer. The scammer will be kind, supportive, and make the victim generally feel like a good person. So long as the victim continues to send money. Once the victim is unable to send money, the scammer no longer has any incentive to provide emotional validation. When the scammer stops making the victim feel good, victims will quickly sober up to the reality of their situation: that as soon as they stopped sending money the love and affection in their relationship disappeared.

If she hasn't done so already, your friend is likely to contact you, and her friends and family, complaining that she can't eat, she doesn't have a place to stay and other woes. Until she hits rock bottom and gives up on "the general", any money you give her is going to be largely sent to the scammer.