I'm guessing from how you put this that it would very much depend on both the femme and the friends. My friends would think I'd suddenly turned into an insensitive idiot if I were to say anything even remotely along the lines of "Oh, well he WAS a girl once" because that's just not something I would ever say or a mindset I would ever have.
With my friends, for example, it would be a simple matter of "well I started dating this guy Bob, he's a transguy who lives in Baltimore, he's 52 and has a grown son in college, and is a programmer with..." and that would likely be the last I've ever need to mention that he was trans - because my friends, and even my family for the most part, know that's how I roll. And sitting them down to explain it would be as ludicrous as sitting them down to explain that he was Buddhist (though of course I would be expected to seriously explain myself if he were a fundamentalist Christian, and my mother would never accept my dating an out Republican.
).
But if I failed to include that one piece of information with my close friends and close family, since it's been 32 years since I've been with a cis-guy, that's when their heads would spin and they would start giving me the third degree. I would actually have to directly and deliberately lie to my friends and family in order to not out the person I was seeing.
And yet at the same time, I totally get and respect the need to be seen and respected as just a guy, and the right to choose for himself whether or not he's out.
As I write this, I realize that maybe the difference --at least to me -- is that the people in my life that I would say that to are people who WOULD see and respect and accept a trans guy as just a guy. And the ones who wouldn't get that, such as coworkers and acquaintances, are people I wouldn't talk to about the details of my life anyway, and I probably end up getting a real kick out of watching their heads spin at the thought of my suddenly being straight in their eyes.