I would have thought this would have been answered by now. The simple answer is yes they can. Had they just told her to go without pay in lieu of notice, then it would have been a bit different. It's their business, their premises, and they have the right to say who is there or not.
Could be any number of reasons, not necessarilly bad
- maybe she genuininely is superflous to needs, or maybe it's just a precation on their part to stop stock going walkies or reputation being damaged etc. I'm not sdaying she would, it's just a precaution automatically taken.
In the financial sector especially it is not uncommon to be told to go at a moments notice, maybe without saying goodbye to colleagues, because there will always be one or two who might want take their revenge in some way. It's just a standard practice.