OK... this seems to be about your relationship with him and how to be in harmony with him while he has another thing going on. Maybe it's a boundary thing you guys could re-map out for yourselves on this one?
I assume you do not want to break up over it. So what do you need to feel safe here?
Relationship grow at their own pace. If he's growing one here with her it doesn't mean YOU have to.
You cannot give your goodwill on this one at this point in time, so need different game rules if he chooses to play here at this point in time.
If you do not trust her, maybe you don't want her in your home for instance? Could he go to HER home? Maybe that could change so you can feel safe in your own home. All safer sex things still apply.
Could address the -- "well, if you are drunk on NRE, am I expected to bring up concerns to you or just stand back and let you deal as it unfolds?" thing. Maybe having a clearer defined role for YOU is what you could need so you can feel mentally safe -- Like you are doing YOUR end of the deal. It isn't anything you did and he's not later gonna go "Why didn't you tellll me!?" because you lived up to your expected duties there.EMOTIONALLY SAFE:
Emotionally? My spouse has told me before "Look, if I don't like them? I'm not crazy about them using you up and they take off and then it is ME holding the emotional bag because YOU are now a mess. What the hell did I do? Nothing! So there's gotta be the plan for THAT. There's being supportive and then there's being over the top with unrealistic expectations of me!"SPIRITUALLY SAFE:
Do you need the plan for that? If you don't like her and there he goes... you shan't stop him. But you don't have to provide comfy later if it goes wahoonie, right? Maybe you need that clear so you can feel emotionally safe and not like he's gonna tap your emotional support reserves later. He went forward without your willing, so he cannot just expect your willing support after either should weird happen. He's not entitled. YOU give your willing when you are ready to.
I don't know what else you could need so you could feel at peace in your soul about it -- maybe time?
Time to see you were wrong (or not?) Maybe she's not as flake as she seemed at first? Then maybe THEN you could give your willing support.
Or maybe she IS a flake. And by being slower with giving your willing, you are still safe and not dinged emotionally because you did not give it prematurely. I don't wish for him to be dinged, but better one dinged than two.
YOU are responsible for your own best healths and well being. So could think about your boundaries on this one with him so your health and well being is preserved.