The notifications "Continue with updated styling" etc,... have to do with which version of the Default.nb StyleSheet Mathematica 9+ uses when opening notebooks from version 8 and earlier. It does not have anything to do with general execution of code from earlier versions. So this is not the issue that you are encountering. The issue presumably has to do with the code itself. But one would need details on the code as well as what you mean by "can't run the script properly". The devil is in the details. Provide them and perhaps the denizens of this forum can ferret out the issue for you.
Memory might be an issue of course, given that your friend's code is taking so long to run. One can often rewrite code to keep track and report on memory usage using functions like MemoryInUse[], etc... And also presumably you can keep an eye on the Mathematica Kernel's memory usage through Windows tools (but I don't know which as I am a Mac user).
