1st question: Is this all the custmer has done with you?
2nd Qustion: Do you have the final art still?
3rd Question: What type of shirt? If it's a junk one, then you know it shrunk with wrong washing.
4th Question: Do you put a hang tag on with washing instructions?
If this was me, amd he did serious repeat, I would make him a shirt, send it to him, order one size up. letter explaining wash instructions and why it's larger.
If he was a one shot, I'd nicely say 8 months later is impossible to remedy. perhaps a better garment next time, at a higher price.
This is a ridiculous incident and should be treated as such. Would not get myself in a tizzy. There will be too many of this in the future.