Obviously that seems logical. Tomorrow if G+ opens ad platform and Faecbook starts advertising I'm sure google won't approve those ads. Its against policy as well, any direct threat to their business will not be approved.
I don't see any reason to think they would change that if/when they bring advertising to G+.
I just searched for "find friends" at Google and got a Facebook ad (among others). Personally, I'd expect Google to have confidence in Google+ and take whatever ad money Facebook wanted to throw their way. I'm sure they reserve the right to block all kinds of ads (including those from competitors). I just think what they actually do is more important than what they decide to keep in their back pocket.