Welcome back! I've been wondering about you. You helped to make my chapter happen, and I haven't even been able to brag to you
I am really sorry to hear that your past president has been speaking unkindly of your friend. I am always the peacekeeper in my chapter (really in all aspects of my life...my family, my job, my friendships, etc...), so I've practically become an expert in tactfulness! If I were in your position, rather than come across as defensive or accusatory (which you actually have every right to be), I would simply say to the ex-pres, "You know that there has been some tension within the chapter lately, and I think now more than ever, we need to all be supportive of each other. We need to approach the chapter with a positive attitude or nothing is ever going to be accomplished. We are sisters afterall, and we need to remember that." If she is openly bad-mouthing a member again, interrupt her immediately by saying "I don't like hearing you talk about our sister that way."
When things get tense (which has been happening a lot lately), I remind my girls that they have chosen to make Beta Sigma Phi a part of their lives, and it will only be has pleasant and meaningful as they make it. When one member is upset with another, I listen to her complaints, and I try to put a positive spin on the situation.
Good luck!
Erica