Why'd my GEN light come on?

I was jump starting a neighbor's truck after it kept dying every few blocks (dead everything, numerous gremlins too) .

I hooked my ground up, then his, my +, and then he hooked his, got a nice pop/spark on his end, shit started smoking on his rig.
Right when it happened, my GEN light came on.
I didn't look at the volt gauge, I just screamed unhook it!

He unhooked in reverse order and as soon as the connection was broken I shut my rig off JIC. Started it back up, all was fine and everything works.

This was a while ago and I never had any problems from it. His car is still screwed but he limps it around sputtering, no anything electrical works except his ignition and headlights apparently.

I'm wondering why the GEN light came on. I've never seen it before except at key on. Not familiar with it's purpose other than telling me when my alt isn't doing it's job (???).

I'm no electrical expert but perhaps he has a short, or another wiring problem that jumped voltage to your rig and blew your voltage regulator? Others with more experience will probably chime in. You could always take your alternor to be load tested to make sure it's good. Perhaps refresh your grounds and battery connections also.

How many times did you jump start him? The gen light means low battery or alt. not charging.
Also, jeep specs?

check your alt

am I reading your post correctly and your "GEN" light is not still on? If it was only on during the time that you were attached to the other battery, I'd say his battery is low enough (or internally shorted) that your alternator couldn't make enough electricity to charge both batteries at the same time.

your light will come on when your charging system doesn't have enough voltage (pressure) to charge your battery-- or in this case, anything connected to the circuit (his battery also)

I had this happen to me a couple years back. It turned out it was a wire going to the power distribution center? fuse block under the hood that had melted. Replaced and all was good. This is just another option as to what it could be. However, like H8PVMT said check your alternator.