Best friend is making a terrible decision. What do I do?

I (25M) have know my best friend(26M) for about 12 years. He’s always been a cool dude and while he has never been the brightest person academically, he’s always been a hard worker and usually displayed solid common sense. For the past 3 years, he’s been in a serious relationship with a girl(24F) I always really liked and thought they were great together.

About 2 months ago, I was hanging out with him and he revealed to me that he had broken up with his girlfriend. I was pretty surprised but in understand that things don’t work out sometimes. What really shocked me was that he told me he already had a new girlfriend(28F)! Basically the timeline was, he broke up with his ex on a Monday, and he and the new girl were official by that Friday. Immediately my alarm bells went off and I was suspicious, he claims that he did not cheat but I find that a little hard to believe honestly.

The new girl is someone he has worked with for the past year or two. I had not met her before and still haven’t up to this point. Since they have gotten together, he has become a total ghost. He never wants to hang out anymore and I can barely get a text back. We did finally hang out again the other day and we went to the pool. He didn’t bring swim trunks, a towel, or sunscreen or anything. It’s kind of like he was already planning to leave early, which he did. He told me that he’s moving in with this new girl. He still has to live at his current place for another couple of months due to the lease so he isn’t fully moved over I don’t think.

My question is, how do approach this as a friend? I obviously want to see him happy but I get so many red flags from this situation. Also, is it really my place to rain on his parade? Idk, I think a big part of this is that he isn’t really acting like my friend anymore. I’ve thrown out some concerns that I have but I haven’t had like a full sit down with him. What would you do in my situation?