Definitely more info please. What is the file extension of the file he's trying to open? How was it created, ie version of software used to create it, name of software etc. What version of Excel is being used to try and open it?
100MB is a large file, Excel is going to require quite a bit of time to open it. 2 or 3 minutes wouldn't be out of the question (longer on a resource starved machine). Just relax and let it do its thing.