How should I respond to my mom?

My bf invited my little brother & I to go eat at Denny’s. He knows I’ve been craving it for a while now. Btw, I’m 17 (18 this year). I asked my mom for permission, she said yes. So some moments later, after I showered, she started cooking & she said “you’re so dumb, instead of going out to eat you should ask him for $100 because the phone bill is due & if not they’ll disconnect the phone today”

It’s like she only sees him as a bank. It’s not the first time she’s asked him for money or told me to ask him for money. He gave her $150 to help pay the phone bill last month & even THAT is way too much in my opinion. She has a bf! He works but he doesn’t make much money & she’s not working atm because my brother is recovering from surgery. I understand we’re financially stressed but it’s also not my bf’s responsibility to pay or help pay things. Am I wrong ?? How should I respond ? Also, my bf is very generous, he doesn’t mind her asking for money but I just feel that she’s taking advantage of his kindness.