Ex boyfriends mom rant

My ex boyfriend’s mom wants to go out for coffee with me. She claims we can still be friends even if me and her son aren’t dating (we’ve been broken up for a month). I don’t know how to politely decline her, so I accepted. She’s moving to Arizona in a few months so I figured 1 coffee/conversation that lasts an hour tops isn’t going to kill me; It’ll be the last time. Her and I got along very well during my relationship with my ex. I used to go to her house for lunch by myself when my ex was underway (navy). If he finds out I’m hoping he knows I’m just trying to be respectful. I’ve never been in this kind of situation before and I have no idea how to end things without offending her. In the future I’ll probably decline the coffee dates from her and speak up about not feeling comfortable due to not wanting to make my ex feel awkward, or feel like I’m trying to force my way back into his life. I definitely don’t want to be THAT ex girlfriend.