Dying alternator?

My friend has a '93 Roadmaster with 44,000 miles and original alternator. During the evening (dark-lights on) hours I noticed his volt meter registering below 13 volts and a slight dimming of the dash lights. I had him to my house and we checked the output of the alternator with my digital volt meter. Without lights it would range between 12.9-13.3 volts. This to me is borderline and in my expereince we should be reading between 13.5-14.5 volts? Am I correct?
The next day he phoned me to say the gauge (which seems accurate) was reading over 13 close to 14 volts. I told him the lights are the biggest draw and I feel with low idle(foot on brake, trans in reverse/drive etc.) I don't think the alternator can handle the load. This seems to be borderline but I think is failing. Tomorrow I plan to remove the alt. and take for testing at my local Murray's auto parts store. Any ideas? Thanks.

Well, i have seen the problem intermittently appear, but mostly with only the lights on, radio on, heat, brake and the directional....
But normally in my 92, it averages between 13 1/2 - 14 in change.
It could be going bad, but i would honestly wait until the alternator dies to replace it (not that it will hurt anything), but it could do that for a month more, or upto maybe 1 year more..
Bill