You gave her the go ahead. So she isn't cheating. But she did LIE about where she had been. That a different kind of betrayal. Sigh. (In my universe
that's a strike 1 offense because SO MUCH relies on truth in communication. )
Also did not give you enough time/nuture/support when you asked for extra help in that area. That all can be chalked up to newbie mono person trying on poly. Perhaps drunk on first time in a long time NRE? Def neglecting ORE needs. And for a monoamorous/monogamous raised person, lying may be the only model they know where they can have both. Does she have enough polyamory reading under her belt?
You could choose to give her a pass with a stern warning not to be so rushy. Esp since you have cut off potential polyships of your own to accommodate some of her needs. That IS her reward for never having lied to you before. A second chance.
Make the plan for how you agree to be together while she's dating and suffering NRE clouds impairing her judgement. It's a hormone high. Not excuse to neglect you or treat you bad, but a reality you both need to address. And lying? That must STOP.
If she's mono, maybe lying is the only way she thinks she can have both of you? We teach others how we want to be treated. If you want her to feel she can come to you with hard truth without you having a cow, after this time?
You have to be open to her making another date with the guy and this time doing it RIGHT with full honesty according to the agreements you made. And you have to receive it without having a cow. She has to be able to trust you too to feel emotionally safe in this new situation of poly dating.
When you make the dating agreement -- be specific.
Are you specific about needing ramp up time to deal with her new dating status? (ex: I need to know about your dates X days ahead of time so I can prepare)
Are you specific about your needs for before care (before the date) and after care (after the date) needs? (I need time before and after your date to reconnect/bond with you.)
Are you specific about what things around the house need meeting? No fair shirking chores/childcare/pet care/bills whatever it is. (Don't know how you arrange your household.)
Things like that.
Loving Radiance had a nice model over there.
Maybe look at it to start the discussion ball rolling as you make your own agreements?
You have to talk it out and make the reconcile plan after getting the full info from both parties on why this happened and where you both could have done better/prevented this, etc. Share the info in a constructive way. Not tearing each other down or playing the blame game. Each one owning their part of the puzzle. Break down that elephant!
Both of you agreeing to put in your own share of this work for healing your SHARED relationship
together. To be able to move forward and back into right relationship instead of all misaligned.
Think about your conflict resolution skills.
Use SMART goal setting
techniques if you want for making your reconcile plan.
Do put some kind of time limit on the plan -- for checkpoints. Maybe once a week? Once a month? To see how it is chugging along on the areas of
1) Do not lie to me
2) Do not get so drunk on NRE time that you neglect ORE time.
3) Keep to our "dating as marrieds" agreements.
Don't leave her hanging open ended like you will NEVER grant trust again despite her work to repair the rift. If you know that right now? Just break up. Spare yourselves the extra work and suffering.
The whole thing of rebuilding trust is a PROCESS, not a single one time event where she goes "I'm sorry" and that's it. And both of you have to be willing to do the work of it.
Hang in there.