Cavalier wont start. Help please!!!

My friends cavalier stopped running about a week ago. It is a 98 with the 2.2 liter. When she turns the key you can hear the starter kick out but it wouldnt turn the engine. I figured it was the starter solenoid, but just to play it safe I took the starter off and had it tested. the starter passed. I put it back on but I left the flywheel cover off, I had her turn the key and you can see the flywheel turn a little bit, so im pretty sure the starter is fine. Im not sure where to go from here, everything seems to be working fine but the engine just wont crank. I can turn the engine by hand so it doesnt seem like its stuck. Any help would be awesome, this is her only car and she needs it running again.

So you can see the starter gear mesh with the flexplate, but the engine does not turn, yet you can easily rotate the engine by hand? If battery voltage to the starter is ok, you need a starter....

I cant physically see the starter mesh with it, but i can see the starter turn it a small amount when we try to turn it on. The starter should be getting plenty of power, I thought it was a dead battery so I put cables on it and used my car for extra power, but it didnt change anything. I was pretty sure it was the starter as well, but how can it test fine at auto zone but not work while it is in the car?

I was pretty sure it was the starter as well, but how can it test fine at auto zone but not work while it is in the car?

Because when they test it, it is not under a load....it's free spinning....

That makes sense. I feel kinda dumb now. I do know that the starter thats in there is still under warranty and when we tested it because it passed we couldnt exchange it. looks like my friend will just have to buy a new one

re-read post #2.....