I`ve got an external app that sometimes adds data to the last row of a sheet, is there a way to trigger this to send a mail with the content of that row?
Try using Zapier :
You connect it with your spreadsheet then if there's a change, Zapier will send you a mail with the content of the row.
Lookup Spreadsheet Row - Finds a row by a column and value. Returns the entire row if one is found.