In the end I know for a fact that I closed one security hole and I am reasonably sure that I didn't introduce any new problems.

There are a lot more people who are reasonably sure that the NMS scripts don't introduce security problems :).

Considering the problems with the creators of the original faulty script your friend was using, I'd recommend you either rewrite the script from scratch or adapt the NMS ones (separation of code and HTML is usually a good thing). If you choose to rewrite the script, it will probably be more work and will result in a less solid script than if you adapted one of the NMS ones.

Your call though, check Re-inventing the wheel is a 'Good Thing' for more opinions on the issue.