From the high resolution version you can actually see that she is wearing Magneto's helmet, meaning he just gave it to her (along with his cape) she didn't make her own. And, from the picture it looks like Genosha is being wiped out behind them. Lorna isn't going to die in this scene, but it very well might be the death of Genosha.
Since Genosha is burning they are likely under heavy attack and there are really two choices Magneto either thinks she betrayed Genosha to those attacking the city or much more likely he gave her his helmet to carry on his legacy (which his helmet represents) and is booting her from the battle because he thinks they are losing and he is willing to go down with Genosha, but doesn't want Lorna doing the same and wants his legacy to live on. Given it is much less likely he would have given over his helmet to Lorna during a major battle if he thought she betrayed him and Genosha I think the latter option is by far and away most likely.
One can make out alot more with the high resolution version of the Marvel trailer up at Marvel.com.