2003 Mazda Protege alternator issue

So a while back the battery light started coming on. It would come on in the morning when my girlfriend would go to work, and then turn off while she was halfway there. Same thing every day. I took it to the shop and had them test the alternator and battery and everything was running perfectly, the light was off etc. When it came back on I took it back to them. Same thing, everything perfect. The guys at the shop called it an electrical issue, causing the light to come on, and "don't worry about it, it's safe to drive." Last night while she was driving home from work, on the highway, the numbers on the clock started fading in and out, the lights dimmed, the radio died and the car started shaking. She pulled off the highway, turned it off and back on. Same thing. Then it wouldn't turn on again. I went to get her, and when I got there she turned the key, the engine struggled for a second and then it fired right up. The battery light didn't even come on! I drove it to the repair shop and just left it there over night, to call them this morning and have her drive up there and give them the keys. Does anyone have ANY idea what is happening? I mean, if the alternator died, it wouldn't start up again right? But the alternator definitely crapped out on her (at least all signs point to that happening). What on earth am I missing here?