Might she be able to? Perhaps depending on whether they note down in the system or not whether the guest has already been 'used' in another lounge, I guess, and then check if it's the same guest if she tries another lounge.

Should she be able to? No. Not unless the person she guests in to the Galleries Club leaves with her, then she guests you in to the Flounge separately, leaving the other person outside without lounge access. But there's no way for BA to easily check the other person has left that I know of.

If it didn't work that way, what's to stop me guesting different random people in to the Flounge, Galleries N, Galleries S, Galleries T5B all on one visit every time I pass through if I could be arsed with the hassle (T5B-A transit included if applicable )? The OW rules clearly say you're allowed one guest only.

The 'correct' answer is for you all to stay in one of the Galleries Club lounges together and be done with it. Or for her to guest one person in to the Flounge and stick together [i.e. leaving together if they plan to enter another lounge together] and you to do likewise to a Galleries Club.