Extremely confused, need some advice (be warned, story's a bit long)

[PFlyers]

Well, now word is getting out that this new girl likes him and he's planning on asking her out :/ Beyond simply not wanting to see that happen for selfish reasons, this girl has a past reputation of quickly meeting guys and dating them (the two have only talked for a couple weeks) and for the last week or so has been flip flopping between which guy she likes. I'm afraid that, besides the obvious reasons to not want to see them together, that she's a bad idea for him and is only gonna hurt him in the long run. How do I bring this up to him without seeming selfish and motivated by my own interests?

I'm just bummed cause this girl hasn't even really necessarily shown any solid commitment towards liking him, while I've taken the huge risk of throwing myself out there and telling him how I feel, yet he still wants to go out with this girl. Over the last day or so as they've become more of "a thing" he's become way more cautious about what he says to me, like making sure to say "dude" frequently, something he never used to, as well as correcting himself on small things (Ex: he said he thought I looked really good yesterday, then quickly corrected himself and said cool instead, as if he was trying to cover up a mistake). He seems fine bringing her up in conversation too, should I tell him to stop doing that cause it really bums me out?

Any advice on this? I also want to thank you guys a ton, you've really helped me understand this whole situation a lot more