This case design is usually refereed to as ref 4240 or 4241, but it is not the standard version of the case whether 4240 nor 4241. It is known there were steel cases from those refs and it is known there have been lug variants.
I haven't still seen enough to authenticate the watch. I need to have a look inside the watch, case and movement to be confident.
But the data seems to be correct indicating manufacture in 1942, without number consistence checking, but that's anyway not necessary if it was bought from Vacheron Constantin. Your friend should got full documentation together with the watch?