While neck size/width can be uncomfortable when playing for the first time, I'm inclined to think that maybe your technique is the problem. My G&L L2000 Tribute is 1.75" at the nut. My jazz bass is 1.5". I play both regularly and have no left hand problems.
Did you warm up (and down afterwards) with gentle stretches before you started playing your friend's bass, or did you just dive headlong into fast and hard playing from the start ? If the latter is true, then this could be what's causing your pain. Drinking lots of water can help prevent cramps.
Assuming your technique is the problem, you might like to check out this link. Either way, it contains good advice.
http://www.youtube.com/watch?v=VRkSsapYYsA