I have no idea how the hell that would be possible in ANY way but fuck it, I want the third party to be the MC's son
Most likely not since he's definitely dead, I think his head was crushed by the vehicle that ran him over?
Anyway, my best guess would be a close friend of the son's who witnessed what was happening (the bullying) but didn't do anything, then realized who MC was and decided to help her with her revenge.
OH FUCK OFF HE WAS THE ONLY ALRIGHT CHARACTER
Literally