ummm why has a non horsey person bought an OTTB

Lol not that that is any of my business

I too dislike horses that get separation anxiety. Will this mare be the only horse in the paddock? If she is then I would suggest that the owners move her to a spelling paddock where she can interact with other horses. If she is with other horses then I would be inclined to have someone there to keep an eye on her for half an hour after you have taken Red away. Sometimes the other horse actually leaving the property settles these dependant horses down quicker than if the other horse can be seen or heard.
Good luck
