If she leaves the dog in the car when she comes to visit - ring the dog warden!!!
I can't believe she is not taking responsibility. The very fact she has even thought of having her dog PTS shows that she does indeed know it is aggressive - silly lady.
Also a muzzle is just a stop gap for now. What about the day she forgets to muzzle her dog.
Many years ago, a dog attacked my Buffy. The owner was full of "she has never done that before"
A month or so later, it attacked Harriet - again the owner stated it had never done it before so I reminded her that was not true as it had attacked Buffy the previous month. I suggested very strongly that it should be muzzled or at least kept on a lead (both attacks happened at the same country park)
Then, one day I saw her with the rest of her dogs but without the one that attacked my dogs. I hadn't spoken to her for months, but had to find out what had happened.
Her dog had been put to sleep after biting a child.
Not suggesting all dogs who are aggressive to dogs go on to bite children, but it can happen.
Hope Willow is ok and that Darren's foot is healing well.
Take care and keep us posted.