Is the customer's keyboard just a numeric keypad or is it a standard keyboard that has a numeric keypad on the right side. From my experience I haven't had an issue with the numeric keypad which is on my keyboard.
SendKeys are generally not the ideal method of communicating with applications due to their focus requirements when sending the key messages. We would recommend to explore a more definitive approach of handling the same . If you are unable to find such an approach, please create an SR so that support can assist with our development team on doing so.