The co-worker has probably worked with the woman for quite some time. Do you believe that he thought she was going to go postal all of a sudden, even after informing him she had a gun?
If she had fired her gun on school premises and then told the police that she had told the coworker, the coworker would have been an accessory for criminal charges and open to civil liability.
The ex-husband likely would not have went after her on school grounds anyway, where there's other people milling around. He'd do it when she gets home.
No. Ex-husbands go after their former wives at their workplace. It is very common.