Slitherine

A situation happened last night where bunches of dragoons were shooting at each other in the flanks sector of the table. Inevitably, mine failed their cohesion tests, went disrupted and then fragged. My opponent then declared a charge on them, and they broke. Owing to the vagaries of dice, and angles of evades, they ended up in a position where his other bunch of dragoons could shoot at them in the shooting phase.
Is this shot
a) compulsory, as a death roll could result
b) not allowed as they cant get any worse than broken?
Couldn't work it out , so advice appreciated.
Tony

Compulsory but not adjudicated.

i.e. They can't shoot at something else if the routers are their priority target, but their shooting against the routers has no effect.

Thanks for that. Very cklear and helpful. Probably will never happen again, but at least I know what to do now.