I just replaced the driver side outer tie rod end--first time ever doing that. Seemed simple enough. However, I'm concerned about one thing. When I was breaking the cinch nut free I noticed that the entire tie rod itself rotated freely inside the boot. Is it supposed to do that? A mechanic told me that is normal but he's a 23 year old beginner and I'm not sure if I trust his advice.

If it's not supposed to do that, what does it indicate? Thanks for any help you can give.

Yes he's right. It is that way so you can do alignment adjustments. You rotate it one way it brings the toe in, go the other way and it brings the toe out. If you have not already, you should go to an alignment shop and have an alignment done after you change the tie rod.