I guess I mean, how does it help her be less spooky?
There are much more knowledgeable people than me posting on this thread, but I thought I'd jump in anyway.
Tying her and leaving her probably won't make your mare less spooky, but it will help with two things: the excessive herd-bonding, and the behaviour she exhibits.
You need to make sure she can't break whatever she's tied to, the halter, the rope, or the snaps if you're using them. If she does, your job just got a lot harder.
Make sure the space is clear around the tying place, tie her securely and all the time with a no-nonsense-no-babying manner, then simply leave her, ideally out of her sight but where you can keep an eye on her.
She will probably have a bit of a melt-down, but let her. She needs to find out that a) nothing she does will change her situation, and b) she'll be more comfortable and less tired if she stops spazzing out about it. So you leave her until she's looking bored and until you're convinced she really is bored with the situation (note: we're talking hours, here). Just accepting it doesn't cut it. Ideally you want her dozing off...
If, after you think she is properly bored, you walk out there and she starts prancing about, turn tail and go away again until she is calmly resting once more.
What you do then depends a bit on her character. You could turn her out with her pony for a bit, or you could take her back to her stall just to reinforce being away from her pony, you could give her some nice scratches and a carrot...
The next time she's away from her pony, or some event happens that she thinks she ought to have control over, she might realise a bit quicker that her having a fit doesn't change anything. It also teaches her that you do. not. fight. with. a. rope
. But this does have to be reinforced with repeated training, even if one good tying-out can work wonders.
I wouldn't recommend the fence option if you can avoid it. Mine ripped a fence post out and went running off with it sliding behind her. And she's not near as big as yours...