I'm sorry to hear about the situation that brought you here.
Often people think some must be dumb, unattractive, or desperate to fall for a scammer's lies, but that's not the case. Scammers, particularly romance scammers, are very practiced liars and manipulators. Whatever the victim is looking for, they will become. They will move quickly and aggressively in order to get their victim emotionally engaged before he/she has time to really think about or question the scammer's claims.
Once they have their hooks in the victim, they often use tactics similar to those of serial abusers. They will shower their targets with romance and affection, then withhold that affection as soon as he/she doesn't do what the scammer wants. The scammer will create a scenario in which he (or someone he loves) is supposedly hurt, and in some way make it the victim's fault, so that the victim feels guilt and a responsibility to fix the situation (which inevitably can only be fixed with money.) Like abusers, scammers will also try to isolate their victims from anyone who might get in their way--including family members. They will encourage victims to lie to friends and family members about their "relationship," and will try to convince the victim that anyone who questions the relationship is against him/her or is just incapable of understanding the relationship.
It is important to understand all of this because the scammer is not going to give up. Once a victim has paid, he/she is a "good" victim and the scammer will do anything possible to keep him/her. After 3 years he has had plenty of time to get his hooks solidly into your mother. And unfortunately, when you came down hard on her, you unintentionally played into his hands. He will continue to try to reach her, and if he does, he will use any tactic necessary. He will try to convince her that you don't understand and don't want to see her happy, and that you are simply unreasonable. He will tell her that it is better for both of them if she just lies to you and pretends she is no longer in contact with him. He will likely try new phone numbers and email addresses, and may encourage her to open a new email address without your knowledge. Some scammers have gone so far as to call the neighbors of their victims when family members have blocked phone access (though this is a little less common in the African romance scammers, which is what you are dealing with here.) If he can no longer reach her through this identity, he will try again using a new identity and a slightly different scam. Some scammers will even pose as the FBI and claim they will recover lost money.
It is also important that you realize that she has committed a crime at this point in time. Whether she kept a penny for herself or not doesn't matter--the moment she received and transferred money (it is 100% guaranteed that the money she received was the proceeds of a crime) she became a money launderer. If she didn't keep any money, that will help, though, as the authorities are less likely to prosecute. But because this has happened, it is best that you keep the correspondence from this scammer, and if you haven't already, file a report with the authorities on the scam itself. This way, if a future investigation of the money occurs, you can show she was not a knowing accomplice. If it is making her more vulnerable, you may want to keep those copies somewhere she can't access them.
There is a link in my signature line for victims of romance scams. I encourage you to read it, and to have her read it also.
If you can post some of the scammer's information here (name used, email addresses/phone numbers, photos, some of the letters he sent) then our volunteers may be able to find more and/or point out other things that help to reiterate that this is a scammer. In a situation like this, the more evidence we can show, the better. It may also alert other women he is trying to scam (it's a guarantee that she is not the only one.)
