I'm not going to pay back the cost of my rehab.
Who spent money on your rehab?
My friend paid for it.
It's wrong to not pay your friend back after they spent money getting you into rehab. Your friend cared enough about you to spend their own money to help you, so you should show that you care by paying them back when you can.
What do you know? Rehab was the best thing that happened to me.
It's always important to pay back money people loan you, especially since you say rehab was the best thing to happen to you. Be sure that your friend knows that you're grateful for the support.