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How does this scam work?
A lot of the comments are about the scam and not about what you should do to help your father.
Make him aware that it's a "romance scam" and that this was set up from the beginning for money. Tell him what to expect and hopefully when this person finally starts with the money requests he will remember what you said and understand for himself what is happening.
Second, the biggest concern would be his inability to get out of the situation and feeling pressured in the moment to give an answer or send money. I don't specifically have a recommendation for how he can avoid this, but generally it's best if people have their own transportation and can leave at any time.
Also, I would stay in touch with him frequently. He is interacting with this person. Talk on the phone several times a day. It's going to be impossible for him to walk away unless you can help him.
Edit: typo