Are you serious or joking? Because, I can still see it as a nice thing to do....
So, hubby and I got engaged inside of 4 months. But it was OBVIOUS we were heading that way from the moment we met each other. Or rather, within 2 hours, when we were sitting at a table ignoring every single one of our mutual friends while getting to know each other. Then having epic 12 hour dates without ANY physical contact, just finding out everything we could about each other.
It doesn't sound like your friend's relationship is like that, and therefore, coming in gangbusters to take over the house like that is generally seen as a BAD thing. I haven't seen the movie with Matthew McCon...and Kate Hudson, but there's a movie about how to lose a guy in x days, and changing/influencing a guy's housing situation, while it's still in early days, is generally something guys don't like.
In addition, if one of us had pretended to be an animal person but then wasn't, it would have been just as big a problem as pretending to want kids. As it is, we both liked having our parents have animals, but WE don't like being the grownups in charge of animals...it's a weird place to be in, but we've both come to it together.
I've had many roomie situations and some with guys, and I've never been involved with any. If any of the guys in my roomie situations had animals that they looooooved, and I knew that their girlfriends were only *pretending* to looooove them, they would WANT TO KNOW. And I would find some way to tell them.
If there is any question about your and your friend's relationship, either by the friend or his girlfriend, is there any way you could have your boyfriend comment to your friend/roommate about how the girlfriend doesn't seem to be a true dog person? I really feel he deserves to know, especially since she's taking over so much already. Who knows WHAT might happen before the truth comes out?
Gosh I hate the misrepresentations that some do while dating! Hubby managed to misunderstand one thing about me...he for some reason thought I didn't have many purses, just b/c I didn't carry one to a big fair on one date...he ended up surprised (I have many, but they are CHEAP, so it's not a vice like others might have), but I never stated that I didn't have purses. However, he DID misrepresent himself on a few things, or just thought I was joking about a few things that were very very VERY important to me, and not only did it hurt like crazy to find that he thought I was joking during Big Important Discussions, but the misrepresentation nearly destroyed our relationship...
Being a dog lover is important to both sides, and I feel it's something worth letting your friend know.