Get rid.
You will end up in the same spot further down the line.
First thing is, she hasn’t randomly started messaging a bloke thousands of miles away, she has either met him before now or went search for something online/app etc. Unless you can suggest how else this happened?
You clearly suspected something was going on otherwise you wouldn’t have checked her phone (and the fact you say she was stupid enough to leave it in the car suggests she very rarely leaves it laying around).
Having a secret relationship, in whatever form, is not acceptable.
Money part isn’t nice, but better being skint and with someone you care about / cares about you.