Disagreed. Wider lenses (of the 28-35 type) are responsible for a very large proportion of great landscapes since Ansel's time to the current day, but it depends on your environment and personal preferences. Mountains account for only a portion of the environment and I find myself using medium wides and short teles in this environment. In deserts, ultra wides and long teles become quite useful.... the sky is the limit. Look at some of the greatest large format landscape shooters and very many are shot with a 90mm on 5x4 (28 equivalent on 35mm) 120/110 (35mm) and 210 (70mm or so): Ansel, Loranc, Sexton, Ross, Westons, Joe Cornish all come to mind as users of wides and teles but rarely anything too extreme. Ansel made good use of the 159 Wollensak (about a 24mm equivalent) in a number of shots but also medium, std and long lenses. Longer lenses tended to be used at distance in the mountains or in open valleys against distant scenes.
I would say for landscapes in 35mm/FF you might be looking at three lenses ideally: Something like this:
24, 35, 75
Seeing as you would be shooting mainly stopped down to medium or smaller apertures, there is an argument to be made for not spending a fortune on Leica glass but looking instead at CV and Zeiss.
Buying a 21 or 24 summilux for landscapes would be a total waste of money and weight. Get a smaller slower lens which costs and weighs less. If you want to go down teh M8.2 route and want Leica, how about:
24 Elmar 3.8
75 Summarit
Together they are half the cost of a 21/24 lux and will provide a medium wide, std and tele. If he moves to a M9 later on he will have a super wide, mild wide and short tele. This assumes retaining the 35 summarit.