Is your customer looking to open the .csv file inside Excel?
If so, you really have two avenues to travel: they've asked you for a csv file and you've provided it. In this case, the next step is their issue to deal with. You cannot reasonably be expected to redesign Excel to work with the file the customer requested. The second option is to suggest a file format that works better with Excel and find a solution for the customer. Did they have a reason for the CSV or did they only ask for it because they thought it solved their problem? Is it being used elsewhere?
If you understand their need, you'll be in a better position to determine which of these two makes the most sense to you.
