I hate how these threads always end up as a debate between two extremes. In most cases, you shouldn't be a rat under any circumstances. In this case, he could be fired for dishonesty because of what someone else did (and ultimately, didn't or wouldn't do). This is that gray area IVE was talking about. Yes, it does exist, Upstate.
In that position, I would've talked to the other driver and asked him if he was going to report it, explaining the difficult position it puts me in if he doesn't: the choice between possible termination for which the union may not be able to get my job back, or being a rat. Offer to go in as a witness if he wants one so they can't claim it wasn't reported later on.
If he doesn't want to come clean under any circumstances, then and only then, report the incident.