I took your first post in this debate to mean that the body only burns fat and not muscle. In fact you did say this and this is what i am asking for proof of because its nothing like i've ever read. So yeah stop making out i'm the dumb one here.
It is possible she is burning muscle but extremely unlikely for a number of reasons. The biggest one is that walking can be powered almost 100% by fat and currently she still has plenty of fuel on board in that area.
My understanding is that walking, like all activity draws its energy from a combination of glycogen and fat after glucose in the blood stream has been exhausted. IF there's no glycogen, in the tissues, the muscles will be broken down to provide it because fat doesn't burn by itself.
According to the usual formula, My body needs 1396 calories for its BMR and i think i weigh less than the OP. I can't remember. If i took half of that away to pay for my walking,, my body would be functioning under par. I might not notice it at first. I might even feel like i'm getting away with it for a while. But i am sure it would catch up with me.