It really depends what purpose it's serving in being a folder. If it's so she can use it on the train, no question from my perspective that Brompton are the best but obviously very expensive and still quite heavy. If you're looking for a cheaper version I'd consider something good but second hand like a Dahon rather than a new cheap folder (e.g. a Globe). If size doesn't matter as long as it folds then I'd go for a 'full-sized' folder which will be easier to wheel along the platform than a smaller folder - best to check with the train line concerned though what they will take and recce how many people take the train when she is going to travel. It's all very well saying they will take folders but if there's barely room to put a person on board, then taking a larger folder will be an issue.
I would generally avoid the cheaper folders that you see on offer in newspapers as the componentry is cheap and won't last long plus they will be heavy and not very effective.
Would suggest you take a look at
http://www.atob.org.uk/It has a good review section.