There us nothing you need to do but listen to her. Many girls get pain when they have their period. She most likely knows what works best for her to minimize this pain..there are over the counter medication, sometimes putting heat on the area helps, sometimes exercise and sometimes just laying down . .the only recommendat ion besides that is if she does have it a lot and it is severe she might consider talking to her doctor.
But if it isn't really there is little you can do but be a sounding board, letting her know you care and comfort to her on the phone or FaceTime ..honestly there wouldn't much more you could in person.