I know what you mean. I'm still on 10.6.8 here and I have no intention to upgrade OS until I buy a new machine. In fact, considering the system requirements for version 11, I won't even bother installing version 11 on this machine.
That being said, if she has a sufficiently powerful machine, it could maybe work - even if PT is not officially supported. For example, I'm running NI's Solid Mix on Snow Leopard even if officially, it requires Lion or Mountain Lion.
But it is quite expensive, as you say. And this upgrade in particular seems to be the object of much thoughts among my Pro Tools using pals. Particularly those who used to depend on the Production ToolKit (which was already darn expensive) - now their only option is to upgrade HD.