I'm going to fake being sick to get out of a concert when my friend bought my ticket
Would you rather be at home than go to a concert?
It's because I don't want to pay my friend back for the ticket
Your friend spent alot of money on your ticket and bailing on him at the last minute because you don't want to compensate him for it is rude. Pay for the ticket whether you go to the concert or not.
I am not doing that. My friend won't trust me since I said I am sick already.
You're right if you do that your friend will never trust you in the future. Just realize that there is no reason to bail on your friend. Maybe just let them know that you are feeling better now instead.