Dating

Would it be unethical to let a guy break up with his girlfriend to move on to another girl?

There is a guy who has shown clear interest in me. However, he has made lies to me or excuses to change plans with me or made excuses on why he couldn’t text back (I realized he was his girlfriend when we were texting). Of course, I did not know he had a girlfriend as he told me that he did not have one. I do not want to be in an affair with someone so of course I cleared up my mind and decided to move on. However, we are in a situation where we still need to communicate with each other and his girlfriend is now out of the country for a while now. They began to be social-media official after I’ve realized he’s been making up lies to me change plans, etc. They do seem to love each other a lot. However, at the same time, everyone has been telling me it seems like he wants to be closer and closer with me, so I am unsure if he wants to cheat on her with me or he’s trying to breakup with her as she is out of the country now. I am unsure when exactly they became officially girlfriend and boyfriend or if they were in a casual relationship and then started dating. However, all I know is that he has been dating her before him and I went out for dinner, lied to me he was single, etc. I have heard of the phrase: “once a cheater, always a cheater.” But, genuinely curious, would it be unethical to let a guy breakup with his girlfriend and move on to another girl?
Would it be unethical to let a guy break up with his girlfriend to move on to another girl?
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