Which way do these windows face ??
For the most part, installing solid vinyl, light or dark, inside or outside doesn't make that much of a difference. If you are installing from near top to near bottom, sun cannot go through and dissipate the heat in the fashion it was meant to be. You can prevent heat buildup, but the vinyl will hold the heat and could easily cause breakage.
Do you want a happy neighbor or an angry one ??
If he can't afford the proper frosted vinyl, tell him he needs to do something on his own, til he can afford to do it...... or make the job into a 'Welcome to the Neighborhood' gift.