[Bluey]

I don't really see that there is a question here that she needs to go asap.
She is getting out of control, you say?
She is making demands others don't use this or that, like pellet bedding?
Wants to use YOUR horse for her friends?

In what world is she living to think any of that is acceptable?
I would have booted her long ago.
No reasons to give, you had a change of mind.
Just say it is your barn to manage and your decision to make, you rule.

[Carolinadreamin']

Boot her. Now. It'll get worse as more time goes by. When she leaves, you'll all breathe a sigh of relief and revel in the feeling of freedom from the stress she created. Don't ask me how I know this.

[Frank B]

Get her out now before she starts running off the good boarders.

[ViewParadise]

I think with people like her, there is no "gentle" way to say things so they get it.

If you have the power to do so, give her a 30 day eviction notice.

THIS!!!! Subtle does NOT work with these types.

[Sunnyhorse]

Any advice on how best to deal with her in a nice way, before I flip out on her and show her the door?

Show her the door before you flip out? (Seriously, she needs to go -- the know-it-all-ness is bad, but the assumption that she could put some stranger on your horse and you'd be happy about it is really beyond the pale.)

[katarine]

She needs to go. now. Whining about bedding is meh. Wanting to ride your horse and getting cockeyed about NO?
Kray, guhl, kray.

[Belg]

Sounds like someone needs to understand the word "coop" comes from "cooperative" and not "coopt".

[FineAlready]

I think I would just tell her: "This is the way things are at this barn, and they will not be changing. No one is going to change their bedding to suit your desires, etc., etc., etc. If it is something you can live with and not complain about, then this may work out. If it is not something you can live with and if you are going to continue bringing it up, then this barn won't be a good fit for you."

Something like that. And regarding asking for her friend to ride your horse...well...it was definitely forward, but at least she asked! Now that you said no, she needs to never bring it up again.

[Carolinadreamin']

Get her out now before she starts running off the good boarders.

Yep.

[KnKShowmom]

Don't know where you are but I think I have had this person's twin in my barn -

Trust me, it will not improve, it will only get worse as they continue to unravel. Save yourself, your fellow boarders and the horses alot of tension and unhappiness and rid yourself of this person asap.

There is no cure for crazy!