To be quite honest, if you were to put all three calibers in the exact same rifle (lets say a Model 7 Remington), and were shooting 140s and a 150 out of the 308, the difference would be almost un-noticable. A lot of felt recoil has to do with the cut of the stock and the recoil pad, although, a great cut and fine pad will never make a 338 kick like a 243, if you know what I mean.

I am a huge fan of the 7-08, and I am nearly as equally fond of the .260. I do like the 308 too, but that 7-08 is just so sweet.

With regards on the direction you should take...if you really think you are going to need that extra bit of bullet diameter that is the difference between .284 (7mm) and .308, then go ahead with the .308. If the recoil with 150's is a bit much for her, go with some of Federals Low Recoil 170gr loads. They will give you about the same trajectory/distance as your .35, with less recoil to boot. When she is ready for the 150s or 165s, you can step her up. But unless you are going after some larger game like elk/moose etc, I'd really go with the 7-08. Besides, the 7-08 has killed piles of elk, some outfitters may express concern, but that depends on the outfitter frankly.

But do make sure you get her a rifle that fits her well, and has a good pad, like Remingtons R3, which is a Limbsaver/Simms pad.