mg zr 1.4 16v - 02 reg?

my brothers car has developed a fault with the gearbox, whilst driving the car, it wnt stay in second gear and pops out under acceleration, every other gear works fine and i have no idea whats causing this, i can select it no problem goin up and down the box, so anyone have any ideas. I know it started this after the bottom engine mount broke. i told him to park it up and leave it but he chose to drive it the 4 miles to his house, could he have knocked the linkage out of alignment or is it just bad luck that the box synchro has gone at the same time. I would b grateful of any advice you could give or what to look for before i go out spending hundreds of pounds on another gearbox and sumone to fit it.