Mare w/ history goes nuts when stablemate leaves

Hi. New here. I recently started working at a private stable. There is a mare there that if we get along in the saddle well, will be my horse. I've spent the last week letting her get used to me around her, grooming her etc. She's gradually calmed down more each day, nickers to me, and today was turning her neck to rub against me as I was rubbing her down. She also set her head over my shoulder and closed her eyes. Always has an ear toward me. Seems good to me, but I never trained in nht, so not sure. The mare has had nht training though for a few months. I always go w/ my gut and am gentle yet firm with any horse I work with.
Her history is one of neglect at one owner, who sent her to a retirement farm for half a year. She is 20. For the past 2 years she's been living at the place I'm now working, had her health restored, but the owner is finding it hard to find a suitable mount for her. Other than her doing some nht, the trainers she had out bullied the mare, and the one rider she had riding her, she said tried to bully her as well. The mare is very flighty under saddle I'm told. She's a Thoroughbred.
The biggest issue, and I think helps feed the flightiness, is that when her stable mate- another mare, is out of sight, she goes nuts. Starts to circle and call for her. The owner said going to a show, or trying to ride them separate is a cause of stress, because the mare is so upset. She also mentioned that because she's not being worked, she's starting to chew on things out of boredom.
I have absolutely no idea what to do. I'd like to go on to groundwork w/ the mare, but if she's so distracted when the other mare is gone, I have no idea what to do. The mares go way back as well. They were both owned by the same owner over 10 years ago, split up for years, then lived next door to each other and would call to each other. Now they are together again. The current owner is thinking of selling her unless she can find a rider that the mare will bond with-thinking this will improve things. She would rather the mare stay where she is, instead of worrying where she may end up if she does sell her.

My best advice to you is that #1- you have to get her current owner to have patience- if you had a long lost freind, and went through her trauma, you woudln't want to be seperate from them either- especially if you are a herd animal and your sense of saftey and security relies on socialization and familiarity.

#2- continue to bond with her, and go for the ground work...ignore the calling. It will be difficult at first- my TB was the same way. The more I work with him, and the more I get him used to the fact that He will return to his buddies once work is done, the quieter he has become...but it takes the patience of a saint!!

She is also 20 and set in her ways. Start small....try to feed her away from the other mare and get her further and further away as she calms at each meal, or every other meal...however long it takes. Take her for very short (as short as 10min) sessions of either walking/grazing or ground work as well. Let her know that you can be just as comforting.

It takes time, is all :)
Best of luck!!

If this mare is spending a good part of the day in a stall, she needs to be outside. TBs like to move, they were bred to be forward thinking. If you have a round pen take a lunge whip, mainly for your protection. Just let her amble around. The minute she starts calling for her buddy, make her move. Ramp up your energy and if she doesn't move, wave the whip in the air. As soon as she's moving, stand still and lower the whip end to the ground. Continue to do this until she turns to face you and gives you her undivided attention. Walk backwards and see if she will come to you. If not, that's ok. Do this daily for a week. You are telling her that you are in command, not her. A lead horse might chase another off a spot but the chase doesn't continue. Never get in near the fence, always leave room from her to run between you and the fence.

Sigh. I feel like I'm up against a time frame, and that's not helpful. I think the owner is going to be patient as long as she sees progress (meaning someone who can partner w/ her in the saddle), not just ground work. I honestly think the mare is also bored, with her chewing and all, and being a Thb. I don't know why the owner doesn't pony her, except that she hasn't ridden her own horse in a few months, and she can't handle the mare. I call her "Red".
The girls go out into a huge paddock (no grass) for about 4-5hrs each morning, and then are in the stalls, unless they get ridden later in the evening when it cools off. They have 2 donkeys with them too. I think both mares need worked, to be honest. It's a little frustrating, because I finally got my groove back, and had everything finished with an hour to spare today. That's an hour I could have dedicated to working with Red and cooling her off etc. But I didn't talk to my boss about it before she went out of town. She gets back tomorrow, so I'm hoping to get working on the lunge. She has two blocked off areas in the turn out-a straight away that is good for working on various movements, and a pen area, though it's not a round pen. To keep an exact circle, I'd have to put a line on her, or she could flit off to a corner and stay there.
The one good thing I noticed, is that when I rub her down, she's started stepping away from her pal, so she can't get hassled. The first few days, she stayed close to her buddy.