Hi bro! You must be opting for the Solo Loop? You need to print this measurement guide in Apple Store Website. After measuring, you can buy it anywhere https://www.macrumors.com/how-to/apple-watc...braided-sizing/

New solo loop need precise measurement for best fit. Can try to call apple and see whether they accept return if it doesn’t fit - unsure whether our region can return easily compared to US / Europe - Anyway, can also estimate by printing the measurement tool, loop it on current watch and estimate the size from there

If he/she is currently wearing a watch, you can use it to help your measurement. Watch will be taken off during shower isn't it?