Cgi, asp, or php?

A friend recently asked me if I can develop a program for her new business. It would be the main software they'd use, and runs all the businesses functions (crm, data and storage, etc)

My first instinct was to use visual c# and create a native windows program. Then I realized creating a web based program would be better: doesn't have to be installed, cross platform, and much easier and faster to design because of html and css.

Now I'm stuck on how I'd do it. I've always used php, but nothing as huge as this project.

Should I use cgi with python, or c++, etc? Should I use asp? Or should I stick with php?

My concerns are security, performance, and reliability. Which of the options would best be suited for a program close to a large native one? If you have any suggestions outside of the options I'm looking at, I'd be very interested to hear.

Thanks in advance!