We have daughters, now old enough to fit adult frames. Decent bikes with decent forks, about 28lbs, that fit them and they feel comfortable on. Their riding has come on so quickly, even though they still claim they aren’t keen (come on! they are teenagers and it’s something their parents enjoy). Rode red route at GT at easter with 2 of them and it was brilliant to see them go for it, even the drop offs at the top. The investment (along with a few years of needing the patience of a saint, tears and tumbles etc) has been well worth it.
In the future, younger blokes may not have this dilemma of how/whether to get their g/f into mtb.