In Computer Science, this type of imbalance (10-20% women) is typical. In fields (like CS) where women tend to be underrepresented, best practices generally say that you should never leave a woman in a group by herself. Isolating a female student can unnecessarily aggravate variety of contextual factors (stereotype threat, impostor syndrome, defensive classroom climates, etc.). I can't say if similar effects happen in other fields, but this seems to be the consensus in CS.