MIL body shaming me and my Boyfriend?

So long story short- MIL and I don’t really get along. And neither does she have a good relationship with my bf. My bf says she’s delusional. For example: her daughter( my sis in law) is gay and has been dating this girl for about 8 years and they recently got engaged. Well, MIL SWEARS that ‘friend’ engaged to her daughter is anorexic, throws up, purposely makes her daughter gain weight and be fat by taking her to restaurants to make her self look skinny and beautiful and that she also fucks guys on the low. PRETTY delusional if you ask me.

Well, we went over recently after not visiting for two months and she goes:

MIL: “ omg you look like you lost weight!”

Me: “ haha yeah a little bit” ( I had lost about 10lbs since last visit)

MIL: “ wow. How did you do it? I could’ve sworn you got some work done. Bbl?”

Me: “hahah no. I’ve been doing a lot of cardio at home and I am on a calorie deficit”

MIL: “ hmm oh really? That’s it? Maybe you should help my son. He’s got a gut on him”

My man: “mom, if I don’t want to diet or exercise tHts my problem..”

MIL: “ you really need to hop on a treadmill or something.. anyways, really though that’s all you did to lose that much weight? Hm cause I SWEAR you look like u got work done. Maybe you shouldn’t lose anymore weight”

After that comment I just laughed awkwardly. As we were driving home I got a text from her saying I need to help her son lose weight and that she’s actually serious, she’s worried about his health etc. Mind y’all, my man has probably gained about 10-15pounds and to me I can’t even tell. He doesn’t seem to be bothered by his weight gain either , he’s healthy af too.

What should I say to her?