If you don't need it, disabling it via the BIOS is fine.
In terms of stability, having it enabled or disabled shouldn't hinder/benefit the stability/performance of a PC. If you're not using software that is making use of virtualization, it should not affect performance.
Are you sure your friend didn't make other changes in the
BIOS in order to try and fix his problems? Maybe he had an unstable CPU or RAM overclock.
I've had virtualization both enabled and disabled and when I've run benchmarks and stress testing applications, I've never seen a performance drop or a stability issue.