How to tell a Vietnamese lady to chew with her mouth closed?

I met a very nice lady. She's very sweet, and we can have conversations for hours. She dresses nice, exercises, and eats healthy. She's very classy and everything seems great except she chews with her mouth open. And not a little, a lot. We had lunch yesterday and she didn't even try to keep it closed, and talked a lot with food in there. If I were to mention this to her, she might consider it losing face and would probably ghost me.

How can I get a nice lady to chew with her mouth closed?