Then I wouldn't put his name on the invite. If you want to invite give him his own invite because you have a close relationship, then go ahead. Otherwise just send her the invite.
So she and her H are back together and she RSVPd for both of them, but texted me and said "he's probably not coming." I told her that was fine and she could bring someone else. Her response was, "Well I accepted for him but he might not show." I have 30 days so it's not a big rush, but I don't think she understands that this is a catered do and I need to give a headcount to the caterer. I realize it's just one person, but it just feels like she doesn't care about spending my money.
You may need to tell her directly: "Friend, my wedding reception will be a catered event, and on (deadline) I need to give the caterer a firm headcount. That means that I need you to give me a firm answer by that deadline as to whether or not you're bringing him or someone else. I will not be able to accommodate someone who shows up at the last minute; nor do I want to spend $X on a meal for someone who doesn't come at all. If you don't give me a firm answer by (deadline), I can only accommodate you by yourself."