IMO, The Truth always prevails.
She asked for it -- how can you be blamed? Hindi mo naman sya kilala, to begin with.
The ex, her current, may call you a Liar -- but Lols, unless he has gone into major reform -- he will run true to form. Who will laugh last? Who is the liar?
IMO, so what if she believes you or not? She asked for it, take it or leave it.
Atchaka, haynako -- don't we all know a woman in love MAY.CHOOSE to be deaf & blind to all?
But nagtanong, eh di sagutin. What do I care what the ex will think? Basta, The Truth ha.
Now, if you wanna go fine print on this -- you may say: As far as I understand what our relationship was... Or, what I experienced was.... Etc etc etc
Nagmalakas loob the girl to seek you out.
Nagpakumbaba sya to ask for your help...
1 human being to another, asking for help -- IMO - sasagutin ko with The Truth, as much as I decide is appropriate.
If after helping her -- Ako pa binaliktad -- fine, it will be just a matter of time before she suffers The Truth.
The Truth always comes out.
But then.. Isn't that 1 good way of Closure? That you can EMPATHIZE with the current GF and step back to tell her what went wrong, what you and your ex BOTH could not overcome, kaya The End kayo? You won't necessarily come across as bitter/regretful naman. Pareho lang kayo decided to search for a better match to your individual selves, ganon lang. You can tell The Truth & still remain friends with your ex. -- Well, at least hindi kayo mag batuhan pag nag meet. Hehe
So, keep out the emotions, keep out the judgments -- just tell her The Truth, as much as you deem appropriate. Like, illustrative example lang ito ha -- Darling, nice to be excited at the edge of a cliff with so much beauty to behold -- but What If matulak ka and you plunge to such great depths? So, you give her guidelines para avoid matulak sya. No need to guarantee tutulak sya, no need to bet she will surely be another Humpty Dumpty. As it is, feel nya na the danger na puede syang matulak di ba? Can you, seeing a stranger at the edge of a cliff, in danger of falling -- can you stand by and say nothing? Can you?
So it boils down to what you perceive is the good and fair thing to do with her request for historical information. This issue of yours -- NO right or wrong answer yan, it's your choice/decision.
Unless, sya yung cause of your troubles with your ex... In which case, tara! tulak natin yan. Lolz.