I didn't think LUL permitted break of journey, hence my question about leaving the station (Amersham is an Undgerground station not National Rail). I suppose that if she had gone out through an automatic gate the system might not have permitted re-entry but that is just a wild guess. The station assistant should be able to override that anyway.

I can think of two alternatives:
1. He didn't actually look at the ticket and, not understanding your girl friend, assumed that because it worked the gates it was only valid to Amersham.
2. He has no idea how to do his job

Complain to TfL.