Do you need a solo player variant to make a Print&Play game successful?
There is nothing particularly exclusive to print and play regarding solo play modes.  They have the same problems as buy and play games.

There are a great many games that cannot adapt to single player, at least not while still being fundamentally the same game.

I would suggest to your friend that he can work on a different game, perhaps inspired by the first, that would play solo, and free him from the burden of making a solo mode that works.

You can check the geek lists at Board Game Geek Print &amp; Play, and you'll see many of the top ranked games do not support 1 player.

As for an optimal number of players, you have averages, but there is no direct correlation between # of players and P&amp;P popularity.  Other aspects of the game (fun, gameplay, etc.) are more important.

A general statement about game popularity (not just P&amp;P) does say that games that require a large number of players (>4) or do not support up to 6 players tend to get played slightly less compared to games of equivalent gameplay, this is only due to the obvious logistics involved in finding a game that fits the number of players you have.