She should be able to do it herself OK - she just needs a passport photo (I think there's a booth in Bermondsey tube, isn't there?) and proof of age (passport, driving licence, or ISIC card), 20quid, and to go along to London Bridge ticket office. Which is pretty straightforward from Bermondsey, so she should be fine.

If I cancel Mon's you'll only get 12.40 back instead of 27quid. Offer to split the difference with her & it'll certainly be worth her while taking herself down there :-)

Anyway, let me know whose you want me to cancel by tomorrow.