Tough call... your decision.
But! Having been through this myself, I can say that it is THEIR rules. Not yours. You have gotten the divorce - and by LAW you both are free to date other people - and/or remarry.
I find it ludicrous that the JWs pick and choose which 'laws of the land' that they want to obey. They seem to 'obey' the one about getting married... but ignore the one about getting divorced.
Anyway... if she is seeing someone - she'll get married to him - regardless - of whether or not she has your 'admission'.
I say tell her nothing. It's her rules - not yours.
Plus - if she wants to get married bad enough - she'll use one of their 'asterisk - fine print' clauses - that allow her to do so.
BTW - I like what others have said about using her rules - against - her - by turning her in to the elders. But... I guess that would be mean. (heh heh heh )