How are you actually feeling about him having this lady back in his life? What rules/boundaries do you have in place? Some people have rules that apply to ex's - I am just asking - we don't. People are generally "exes" for a reason - have any of those reasons changed?
Do you actually have the contact info for this woman? Or would you be dependent on him to provide this? Is there any provision in your relationship with HIM where he has the right to deny that you talk to anyone that you want to? If so, did you agree to this?
It would be helpful, if we are going to be of assistance, to know what transpired previously that caused things to "end poorly on everyone's part". (i.e. did the three of you try to make a "triad" that would have been better off as a "Vee"?)
To me, it sounds like he is feeding her "lies of omission" (technical truths which don't answer the spirit of the question) - which makes me suspect that he is not being entirely honest with you either.
While I do not insist on meeting my metamours, if they are uncomfortable, it is a "red flag" to me that he feels that he is in a position to "allow" you to deny you permission talk to whomever you want. (Do you have a d D/s dynamic that you haven't revealed?)
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (21+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
TT: poly male, married to Lotus, FB with JaneQ
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe