Very
That’s a big old “it depends.” UC is 590; Columbia is 780.
Another “it depends.” Usually it counts as fulfillment of the language requirement but not double-counted as a gen ed credit for humanities; often the credit is assigned as elective credit, but it varies.
Even if the college accepts AP Stats credit, it is very unlikely that he can apply it to his major. Stats for STEM majors is calc-based, which AP Stats is not. Given the choice between focusing on the AP Stats exam and the AP French exam, I’d focus on the latter, although TBH the amount of prep needed for a strong STEM student for the AP Stats exam should be minimal.