I'm thinking of encouraging my friend to go all alone to meet her ex in another potentially dangerous city without any of her parents knowing
Why would you encourage your friend to into a potentially dangerous situation? What is your motive?
Money, I'll make money for this by selling her a plane ticket.
It's good to want to make money, but you shouldn't do it at the expense of your friends safety. Encourage your friend to bring someone with her instead.
I can't go, I am busy.
If you cannot go, then ask another friend. Or you could even ask the parents of the friend. There is no way you can let her go alone in good conscience