If it got it in Pokemon Black, it's not legit. There has not been a Mew event since Gen IV, and Mew is an event only Pokemon.
However, if he transferred it to Black from an older generation, then it's possible that it is legit (Mew events given out in Gen IV were all in Cherish Balls).
To be honest, if it was legit he would have told you he got it from an event. The fact that he's not telling you where he got the Mew makes it seem shifty.
Mew - events