We took K to walmart and let her try on bikes. I told her I was thinking about getting her a new one when I could afford it and wanted to see what size she needed. We selected the one she could touch the ground with her tippy toes when the seat was in it's lowest position. That way as she grew the seat would come up and she could still touch her toes for a long time.
You could say your friend wants to get their child a bike and they want you to help figure out what size they need OR you could have a friend take him and say that they are getting their kids bikes for a surprise a need someone to test the size for them. It's still far enough off to Easter they won't associate the bike fitting to the Easter Bunny.