Ex seems to be spiraling out of control. What do I do?

I've noticed that recently my ex seems to be spiraling out of control. In the span of a month since our breakup she has literally become an entirely different person. Most recently my ex posted a picture to social media with her leg around some random guy.

Would it be out of line for me to express my concerns to her best friend and request that she makes an effort to keep my ex safe?
Ex seems to be spiraling out of control. What do I do?
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