Just buy the game.
I think he's already got an account, this may be useful for a school or something. Setting up 100+ mc accounts are hard, but making a login server for the Windows Login and Minecraft Login could be interesting. (Yes, I know Minecraft in school is kinda a cheesy concept, but don't act like you didn't think about its educational value
Personally, I think it'd be cool. Figure it out, Longor!
You have to replicate the ENTIRE Account-API 1:1, and it takes a lot of time to get everything right.
If you don't get everything right Minecraft will, most of the time, just crash.
I tried it, and it absolutely wasn't worth it. It was actually the worst idea ever.
One had to install the 'patch' on every Client that wanted to join my modded Server, and since people are too lazy to do such a thing, I lost all players on my server. ALL of them.
It usually did go like this:
- User tries to join Server.
- Client crashes.
- User says: "Stupid Server, makes MC crash! I will play elsewhere..."
- My Server got more bad reputation.
- GOTO step0.
Tl:Dr; Don't try it, you will loose (nearly) all of your players.
Have a nice day.
Edit: By the way: I stopped that project the moment someone 'higher up' told me that what I was doing was against the EULA. Don't ask for the code, I don#t have it anymore anyway.