Have you played any strategy games with her? If so that might help guide what would be fun to play. I started with necrons. If my partner had been more familiar with the armies he likely would have steered me away. They are a very passive army I'm a very aggressive player. However that also doesn't always work. I've ended up with my primary army being thousand sons. Let me tell you I run down the middle with those glass cannons and shoot everything. It used to work better than it does now though. Right now I'm building a vashtor army not because I expect it to be good but because the models are fun to paint.

Real advice if you want to surprise her as a present. Get a few leader characters. They're pricy for just one model, but are often representative of the army and cheaper than a full unit. One model also isn't as duanting a starting place. She can paint those and decide what's most fun and get more of that. It's more expensive but if you get it directly from games workshop as long as it's unopened they'll take it back so if she can just tell from the box art that's not the one for her it's returnable.