Did your friend actually get into an accident, or are you just worried about the "what if"? That makes a big difference legally.
You're in a decent spot since he had his own coverage when stopped, but I'd push back on the earlier answers a bit. It's true that insurance typically follows the vehicle, not the driver - your policy would be primary if a claim happened. However, the critical issue is whether he was a permitted driver under your policy. Most policies automatically cover household members and occasional borrowers, but some don't, or they have restrictions. If your friend was specifically excluded from your policy or wasn't a permitted user category, your insurer could deny a claim entirely, leaving you personally liable for damages. That's the real risk.
The fact that he had his own insurance is helpful but doesn't automatically shield you. If he caused an accident, your insurance would likely still be the first payer (up to your limits), and then his would be secondary. But if your policy has a gap or exclusion, you're stuck holding the bag. The broken taillight stop was minor and worked out, but going forward I'd recommend asking borrowers directly if they're insured and maybe adding a quick call to your insurance company to confirm your policy covers lending to friends - takes five minutes and saves a lot of headache later.