As others have suggested, she may just be bi. [In fact one theory has it that all women are bi to some degree or another.] And while lesbian behavior does not meet with great social approval, it is not nearly as objectionable as gay behavior [possibly because of a widespread assumption that woman, and what they do, is not as important as what men do.] In any case, women can behave very lesbian in public and be ignored.
As far as the boyfriend is concerned, there is the lesbian t-shirt "I fxxxed your girlfriend, and no, you can't watch." He would probably suggest a threesome, but overall, he is not likely to be too upset, at least if he gets some too.
So keeping the matter secret is likely fine because nobody is going to be that upset when it comes out. You might suggest she be open, by unless you know otherwise, you should assume the others are not too worried about the point.