Dude, girls CANNOT measure ANYTHING acurately, they have no sense of size really, most probably she is just telling you that he had a 10 incher because that's what he told her ( very likely a lie ) because he lied to her, men usually "inflate" their real size for some kind of strange sense of manhood, or...just to impress them since we men know that they are terrible at gessing sizes.
A 10" dick is certainly possible and thousands exist in this world, but it is VERY VERY rare, as in 1 in 100.000 men and just maybe even rarer than that. ( can't remember the percentile on that )
Hell, even 8 inchers are VERY rare let alone a 10 incher.
Just look at something 10" long, put it in front of you where your pubic bone is and just look at the ABSURD lenght that it reaches...Not probable huh, in a World where average has been put at 5.5 inches lenght, a 10 is what? Twice the average almost? Do you think that your girl used to date a guy that is almost as big as the biggest dude in adult entertainment? ( mandingo ) just think about that, discard it as only a fantasy ( probably ) and don't stress about it, get to work and build your own HUGE DICK.
Peace
P.s. Don't believe everything people tell you or everything you read...most often reality is quite less interesting and more down to earth.
Mike