You get what you pay for I'm afraid. Halfords bikes are covered in cheap parts that work poorly and fail often. The one I had purely for commuting ended up having another £150+ spent on it after the event just to keep it on the road. A complete false economy.
She should spend what she can if she wants to get properly into it.
I trained for my first Marathon last year while working shifts on ambulances, much like yourself this is very variable from week to week and it is nigh on impossible to do a decent run after a long day/night.
I gave myself a target of an average 3 or 4 runs a week then made a running plan accordingly. I built in some flexibility to allow me to move things around if I was particularly worn out and this helped.
Some weeks a ran loads others maybe just once or twice....