I've read all the replies, which I found interesting, but here's how I see it. OP, you as the permanent employee should have priority over a temporary one, especially one who works on a part-time basis. But ... I think it would have been courteous of you to have waited for the employee to show up, then gone to her and told her that in accordance with your needs and your boss's instructions you needed to switch chairs. A brief but sincere apology for putting her out would have helped ease the pain for her, I am sure.
Temps can be treated like garbage and lied to and they have no recourse. It doesn't happen all the time, of course; some agencies and their client-businesses treat them well. But there are others where, well, let's say the temporary employee is treated as disposable, and give no respect at all. I saw that more than once, and it's the reason I refuse to consider ever working with a temp agency again.
In this case, the temporary employee was given no respect, not even in the form of simple courtesy. I think sympathizing with her feelings, even if nothing changes, would go a long way to making things better for her.