If he is coming in from Switzerland he will land in the Schengen (EU) side of the airport and need to go though passport control to international side of the airport. If you have no idea what I am talking about Google Schengen AMS.
FYI, when you get to passport control from Schengen to international be an ******* and tell the guys you have a tight layover, they will move you to the front of the line.
It is better to be aware of this before you land, take your piss before you land and not before you get through passport checks. I have seen countless Americans flying though AMS who screw this up.
One more edit. YES, a majority of the smaller regional planes will de board on the tarmac and then bus transfer to terminal. When you come off of the bus you are literally right at the immigration check. Once again, it is doable as long as you are aware of what lies ahead. Get on bus, go to terminal, get to passport control, tell them you have tight connection and get to front of line, and then go directly to gate. It is confusing first time as this is different than US airports.
Worst case you miss your flight and you are in Amsterdam city centre in about 20 minutes having a beer