Warranty has nothing to do with ban.
Warranty is lost anyhow when you open your console
The warranty sticker tears off.
Getting banned is possible If the Modification files are old and u try playing games online.

So here's what u need to know if u mod ur console.
1. Update to the latest dashboard. (2.0.13599)
2. Mod with the latest CFW ( ixtreme LT + 1.91)

If u did both then u r 100% secure.

Ur friend has just the rough idea. That why he don't know exactly what he's talking about.