However, this is obviously not going to happen and I can see by your posts that you dad loves this horse and is going to provide her with the best care he can.
I have seen one injury that is reasonably close to this. My coaches young WB put his leg through a fence kicking out at the neighbour's horse. He skinned his hindleg to the bone in some places. We thought that was the end of him.
Shuttled him off in the float and sent him to the nearest equine hospital. He was there for a week on IV fluids and painkillers, along with sedation to keep him still. Like your dad's mare, there was nothing to stitch to.
He was put in a full leg cast that was packed as tight as possible with gauze and elastoplast. Then stabled for almost 6 months. It was a terrible process, but he was such a great patient, he seemed to know he was being looked after. It cost my coach upwards of $2500 to treat him, but he was a $50 000 as a 2 year old so she wasn't going to give in that easily.
He is now out competing dressage and you wouldn't know other than the scar. Not a hint of lameness on it and he has started collection without an issue.
However, his injury was at the front of the leg and his missed the tendons. Once a horse has lost tendons, then you will run into huge problems.
Your dad's mare will most likely be permanently lame, and also prone to arthritis. I would say she'll need regular visits from a chiropractor/physio if she pulls through, as she will also be walking unevenly on it which will easily put her back and shoulders out.
This is going to be a VERY expensive long term injury. She should also be on painkillers, which in the long term will have detrimental effects to her health if she is on them for an extended period of time. It is a huge gamble you have taken.
I would tell your dad not to rebandage and clean it every day. You want it to settle and promote growth. I would literally drown it in white ointment (a thick paste that is an antibacterial, healing agent that also keeps the flesh soft to promote healthy tissue growth), then pack it with gauze and elastoplast the whole thing quite tightly. Keep her as still as possible. I'd keep her off hard feeds and just on meadow if possible. Wrap all 4 legs for support with well padded bandages (wrapping legs over a long term basis can lead to bandage bruising. Not sure if you've seen results of bandaged bruising but it can be pretty horrendous, google it if you're not sure).
You want to keep the flesh as soft as possible, if it scabs over it will make healthy tissue more difficult to grow. Keeping it soft will make it take longer to heal, but it will heal better than leaving it to harden.
I would be cleaning and re-bandaging every 2-3 days. Being extremely delicate with the process as you don't want to knock off the highly delicate new skin tissue. You are going to have an absolute hell of a time trying to encourage the new skin to grow over the bone. If there is no 'bridge' of tissue for the new tissue to grow along, then the tissue essentially 'gets lost' and doesn't know where to close up. That is where skin grafts come into play. They create that 'bridge' of tissue to encourage growth. However, this mare's leg is so badly damaged, I highly doubt they could graft it.
Proud flesh you don't want, it will inhibit the healing of healthy tissue.
Keep the exposed bone covered at all times! Her stable should be sterilised. Once a week completely clear it out and disinfect the whole thing, then put new bedding down. Everything has to be as clean as possible. CHanging bandages, wear gloves, don't let new dressings touch the ground etc. If she gets infection in that bone you will be in BIG trouble. Once infection spreads to the hock there is nothing you can do. You would be forced to put her down, the hock joint can degenerate and be eaten away by the infection, which at the very least would leave her with horrendous arthritis in the joint, making it disgustingly cruel to keep her alive.
I also worry about the lack of tendon's left. If should does recover with the wound closing up, she's always going to be lame on it. The tendons hold muscles to bone and without them, she will always be weak and sore on that leg. It is possible, with IMMENSE expense to attempt to repair damaged tendons with surgery, but again, I highly doubt this would be possible on an injury of this degree.
I wish you and your dad best of luck, she sounds like a very special mare to you and I hope she recovers. I also hope you have plenty of money to spend on the recovery process. Bandages cost a fortune. I suggest having a look on Ebay and buying bandages and gauze in bulk. It will be cheaper and my god, you will need it in bulk.