In a nut shell: He had the wrong plan A, but the right plan B ;-)
Happily for him, it did work out in the end.

In the Alps, wave with northerly winds is typically stable situation (it can last for a week), and more or less dry. It's a large scale situation controlled by a high pressure zone. If you want to descend in IMC, you just need to make sure that you won't hit an embedded rotor. Ceilings will most probably be where they have been hours ago.

Wave under southerly winds is controlled by a depression coming in, and has a time scale of something like 12-36 hours. With the depression moving rapidly closer, wind direction and moisture can change rapidly over the day until finally the warm front cuts off everything with rain. A descend in IMC without solid information about the clearance between ceiling and ground is just Russian roulette.

Bert
Ventus cM TW