Engine trouble-won't start

My friend has a 97 or 98 Blazer with a V6 Vortec. I'm not sure what liter. Last night it was below 0 in Massachusetts, and his car wouldn't start. It would only give 1 click when he turned the key. His lights and everything seemed to work fine, so we figured it wasn't the battery, plus the fact that it only clicked 1 time. We tried to jump it anyway, and even let the battery charge for a few minutes off of my Dakota battery, but it never helped. His vehicle had been running just 5 minutes before. What do you guys think....alternator? Starter? I appreciate the help...