In reply to holysock: Eh?!?!?
Loads of rucksacks have ski attachments suitable for ski mountaineering....especially as your mate will spend the huge majority of time with his planks on his feet rather than on his rucsac. I'd go so far as to say it would be hard not to find a pack with ski attachments on.
Have a look at climbing packs in the 30-40 L bracket made by Black Diamond, Osprey, Grivel, Lowe Alpine, Berghaus, The North Face, Mountain Equipment, Aiguille, Pod, mammut, Marmot, Tiso, Haglofs, Salomon, OMM, Crux, Macpac, Deuter, Ortovox to name but a few........