If Wickham thought he had a friend (or two) in London to help him out, then he just needs to get to London. Lydia might have had enough cash for that.

A possible scenario (my speculation only): Wickham could probably see that Lydia was willing to attach herself to whichever man was currently paying her the most attention. Maybe he noticed that she also had a little money, so he started working on her to see if he could get it. Then he realized that if asked to borrow some of her money (enough to get him to London, for example), he would have to give her a reason and she might tell Mrs Forster, who might then tell her husband who might catch up with Wickham before he got very far. If Wickham gets Lydia to think that he wants to run away with her to Gretna Green to get married, however, she would probably agree to keep it a secret. So, he concocts this plan and gets her to pay for the travel, and he gets a travel companion (which might be better than travelling alone).