iPad Air and External SSD

try a different cable first - one specific for the ipad connector socket
you also need to rule out any hardware problems with your friends ipad

also get back to square one and document more clearly your fundamental requirements rather than provide a solution/constraint. Do you want the external drive to be readable and writeable by both devices - why? File/data share? Any other way to do this? Why is it anyway you wish to belong to two different eco systems? There can be many sound reasons, but it helps I find if you can properly write down why --- sometimes it can better inform you whether you should in fact simply stick with one eco system rather than mix and match. (edited)