@gw0gvq , Hi I created a short < 2 minute screencast 'tutorial' on how to convert a Numbers spreadsheet to Excel and then email that to your Windows User.
I chose Apple's Numbers spreadsheet as it is simple to use and free for Mac users, and it is relatively straightforward to export to Excel format for simple spreadsheets.
Numbers conversion works pretty well with simple 'one tab' spreadsheets and tables to Excel. If you plan to do anything more complicated then you are probably better off signing up to
Office 365, which is Microsoft's subscription office suite for proper compatibility with your Windows friend. Office 365 has Mac versions of the main office Apps, including Excel.