The person you're responding to has a ton of Fatlogic in it.

OP says the GF is walking and restricting calories, and this person blows it up to skipping dinners. Another response asides from yours infers that the OP's GF could develop a full blown eating disorder.

It's useless arguing with these people because there is no middle ground with these people. All they understand, is that the way to combat being overweight due to excessive eating and sloth, is to excessively not eat and run a marathon every day. There is no such thing as moderation, being logical, and reasonable.

The OP says that the GF is walking, and restricting calories. Where did this whole cookie/dinner thing come in? These fatlogicians interject fact to villify the narrative.

Is the mom really that bad? The mom doesn't see the kid for some time and it's normal for a parent to say yeah, you're fatter. It's blunt, but it's her mom. It's just more spoken than some random friend that she hasn't seen for a year.

Good luck arguing with these people. They have an answer for everything if you allow them to add to the narrative and ignore logic and science.