She deserves the respect of you being honest with her. Tell her you value your friendship with her, but would rather you two didn't discuss your ex. If she finds this impossible you have to respect that and agree not to see each other - at least until you feel more neutral towards your ex.

To be honest I don't really think you should, its sort of in the situation I am in. And I'm the one the ex-partner, and I feel really horrid knowing that the two of you kept in touch without me.!