Just to provide a little balance; at a ground course some years ago I was introduced to a British man who'd flown many years for an airline in the UK. He had something like 9000 or 12000 hours (I don't quite recall the exact number, however it was in that region), but had been an FO for all that, and other than his initial training had no PIC time.
He was a pleasant man whom I'd got to know, but during the course (on a type he had thousands of hours on) I was surprised to find him struggling with some things, and subsequently I don't believe he flew the 'plane this course was for.
Although, as with the subject of this thread, this is just a sample of one I suggest that he'd never held command for good reason. With regard to Ms Forshaw it's always possible that if she has wanted to upgrade, she may not have managed this for legitimate performance or other 'luck of the draw' reasons that are not gender related.
So, if lack of advancement forms part of her complaint (and I'd want to see the complaint verbatim, secondhand from a news article can lead to misinterpretation) I do hope the investigation is as dispassionate as possible on this matter, and is preferably conducted separate to any harassment matter. I should think that mixing the two would make it very difficult to differentiate where needed.
FP.