Difficult one. If she has a good relationship with her boss and his wife, then I can't see a reason not to accept such a lovely gift.
However, the company may have rules on receiving gifts - especially expensive ones. Not having seen the pendant, it may be that it's not especially expensive. Maybe everyone is thinking in £000s!
If her relationship with her boss and his wife is not that close, it may be wiser not to accept, saying 'thank you for such a kind thought, but I couldn't possible accept such an expensive present.