You're in an interesting position Grogs, and I don't envy you.
Based on how Bec took the situation with the first girl, I'd tell her. It may be rough, but I as I see it, there are three possible outcomes:
1) You get lucky, nothing happens with the French girl. I think that this is probably a little sketchy. The more you don't want it to happen, it probably will.
2) You tell Bec nothing, and she gets surprised, and everything goes to hell.
3) You tell Bec everything, she may be a little jealous, but you assure her that what's done is done. and you're commited to her. She has an idea of what coulde happen, but anything that does won't be a huge surprise. She meets the French girl, nothing bad happens, everyone is happy.
I see not telling her anything as the worst possible situation because then you run the heavy risk of have her find out, and having to explain everything on the spot. This can make Bec angry that you didn't tell her, and jealous of her. If you tell her ahead then you have a certain degree of control over the situation. You can explain everything with a cool head, and without having to worry about damage control. But then again, I don't know Bec, and that counts for a lot. But from my perspective, I'd hate to have to try and control an awkward situation that could be easily avoidable.
Good luck! :thumbsup: