First off she is going to really miss the other horse. Can you house it as well? I gather you don't have any pasture so you will need to exercise her several times a day, either lunging or riding. Get 2 small mesh hay nets and hang them at opposite ends as this will encourage her to move back and forth. Let her soil in there, pick it up and that's where you want to put down straw. You don't have to bed the entire 30'. The stones will strengthen the soles of her hooves. A straw bed, 12'x 12 is plenty if she wants to lie down. Since she will be somewhat cooped up don't feed any supplements, just hay.