Can I take legal action because my ex who is 15 is dating a 23 year old man?
It's definitely illegal for 23 year olds to fuck 15 year olds.
I'm also mad because she broke up with me for being boring.
I'm not sure your motives for action are pure here. But it's still wrong for them to be dating. I think it's okay to tell the authorities about this.
They are both adults and they can do what they want.
They aren't adults if she is only 15 and what he is doing is illegal, so you should definitely tell someone. It would be good for everyone to break this up before someone does something that they regret.