It actually depends how the rules are applied. If the lounge was a CO Presidents Club one, she would be allowed. From CO.com:

BusinessFirst and International Business Class customers are also welcome to visit the Presidents Club*. A same-day BusinessFirst or International Business Class boarding pass is required for admission. BusinessFirst customers can also bring up to two guests, or a spouse and children under the age of 21. Domestic First Class customers do not qualify for admission.

The fact that it is a shared lounge in LHR may mean that this doesn't apply though.