Perhaps we could find someone in London who could go with her to meet this person, just in case.

I think you're just being a good friend, and being cautious. She's not taking into consideration the risk of being somewhere you're not familiar with, where you don't know who to call for help, if you're lost you're really really lost, if your passport/wallet is stolen, you'll be at more risk, etc. This is a rebound relationship for her, so she's probably just not thinking clearly, and not wanting to hear anything negative.