Reading through your comments, I think there's really only two resolutions to this situation. The first is that your mother in law will run out of funds, and die penniless. From what you say, that seems to be where this is heading. However, the only other resolution I can see is that she realizes on her own what is happening. You can lead her to the water, but you can't make her drink it. Realistically, I see that happening in only one of two ways. Option one is that he admits to it being a scam, and cuts contact himself (or gets arrested for it). I don't think that's likely. However, option two is, as others have said, she could try to plan a surprise (or even a non-surprise) visit. If you propose this, I would recommend you do so in a positive light - don't say anything along the lines of "well, if he's real, he should want to meet, right?". Rather, phrase it as the next logical step. It only makes sense that two lovebirds should meet, after all, so she should accept it pretty readily, if you propose it as what's the next best step for their relationship.
The meet solution doesn't always work. My father has met a woman whom his scammer claims to be twice. I'm quite sure it's a man behind the emails and this woman has been coerced into meeting him.
