Might be best to give your friend a pair of good fitting slippers to take with him.
If you intend to wear the clogs for serious periods, you have to allow for some really thick socks. They will feel very uncomfortable to unaccustomed feet.
The proper way to wear clogs is with a special pair of very light soft slippers. Make sure that your friend doesn't go for the cheapest ones, as they are made to hang on the wall, not for wearing.
It would ofcourse be so much better if you could pop over to Amsterdam yourself to get a perfect fit. A proper clogmaker is a real craftsman and will ensure you will be fitted with the right arch, instep and sole width. Worn properly, you won't believe how warm they are out in the freezing snow!