Of course the employee can fight it. It's his right, but he'll be darn stupid to given the circumstances.
There's nothing to indicate that there was anything wrong with the vehicle. So saying the bakes could not have been serviced is just fishing.
In all likelihood he'll lose and then he'll have to cough up lawyer fees as well. So if he's smart, he won't be a hard-ass about it and pay for his mistake.
I wouldn't be so quick to make knee-jerk judgments when you don't have all the objective facts. By asking for the objective facts, you are not being "darn stupid", nor "hard-ass". He is perfectly entitled to enquire whether the company vehicle was properly serviced and maintained. Surely not being able to stop in time is all the indication you need?
And what about road conditions? How about visibility - was the morning sun in the employees eyes?
Was the OPs brake lights working? Was it on a blind corner?
Your keyboard strokes wash right through these pertinent questions in your rush to presume liability (not guilt), and 100% liability at that.