Within reason, yes. Just be sure both positive cables are connected, and then connect the grounds to something metal under the hood, (to keep sparks from the battery.) The only was I can see to "blow" a computer is to cross the leads. You do essentially the same thing if you connect a battery charger to the battery while it is connected to the car.
I truly don't see how he got a computer, unless something was crossed, or he had a problem already.