I'm probably missing something...but isn't that what the quote characters are supposed to solve? If you put the entire field in double-quotes then Excel will treat it as one field. It looks to me like you've deliberately turned off the feature that will help you.
Quotes are the mechanism that CSV files use to cope with commas - I don't think Excel is going to understand anything else. If you really can't have them, I think your only other option would be to remove the comma first - replace it with a different character.
I think you're going to have to explain to the client that Excel and Notepad work in fundamentally different ways, and the file cannot be generated so that it behaves the way they want in both of them. They're going to have to choose.