I completely disagree emphatically with you or baby G telling current wife anything. Do either of you need that on your consciences for the rest of your lives? I think not.
I think your ex, you and baby G need to address this in a therapy session and after the three of you do that, then maybe his wife can be brought in and he can tell her in front of little G and the therapist.
We have to protect little G and think long term here. She doesn't need to tell anyone but her dad what she saw. She doesn't need the guilt of whatever will happen in that marriage once the wife finds out. Yeah, I know it's not her fault, but she's a 14 year old kid. She's def gonna feel like she has some responsibility if she's the one telling the wife. Leave her (and yourself) out of that rathole. this is his $h!tshow.
M 20+ T25+ S ~15.5 (BD) BD 4/6/15 D 12/23/16
"Someone I loved once gave me A box full of darkness. It took me years to understand, That this too, was a gift." ~ Mary Oliver