do i tell this girl my ex two-timed us?

so there was this guy i dated for like 5 months last year, we broke up october 19th 2020. we talked almost every day through january, and he was still flirting with me through december or so (and giving me false hope). he started dating someone new in december i thought, but turns out his new girlfriend posted today that they've been together a year (meaning they started dating october 21st 2020). clearly he was talking to both of us at the same time and i could care less because i think he's trash anyway, but now this girl has been dating him for a month (presumptively) not knowing what was happening. do i reach out to her and tell her? her story was sent to me by a friend, we don't follow each other.