Just brace yourself: know it is coming, that it is a problem of hers, and don’t get too much into it when she makes her riots.
If you can, talk about it when she is not PMSing and agree on a way like a code word, or something that you can say that will remind her she is in her own state and this is not the time to talk over things. If she is still upset after her menstration, she can talk to you about her issues then.
As for yourself, you can make these days the days you see friends more often, days you take for yourself and try to avoid any contact that will stirr things up (be more sensitive in those days).
Another thing you can do, if you want to also help her, is to listen while she gets upset: just listen to what she has to say, how she feels and how she sees things, you can ask questions like “what makes you feel this way?”, “how would you like things to be?” and so, just to help her get the venom out.
You should know that the PMS is a response to emotional stagnation, in this cleaning time of the female body, stuck emotions come out aswell, and all she needs is to get it out. Taking it out on you is not “right”, but you can also help her by helping her get rid of this “dirt” she is holding, and you will see each time once she gets it all out (once she has no more complaints or anger she wants to communicate) she will feel much better and will be loving and thankful.
Basically, just know it is coming, accept it (don’t try to change it, or her), and act accordingly.