Assuming the best intentions, they could fully intend (and ideally be in the process of) returning your charger. They could just be asking to ballpark when your phone will be at full charge so they can ask to use the charger again later
There are still areas where one has to share one charger? Hell, I probably could build a house with all the bricks I got thrown in with some device or other. A sad, plastic house, but still.
With iPhone (finally) switching to USB-C this will become even more of a problem. I used to pull out my old iPod charging cable from storage when a friend would ask me for a charger at my house to get a rise out of them. However my portable charger did come with both USB-C and lightning connections so they were always in luck.