Speaking as a female, it depends. We're all --
gasp! -- different.
Women on the pill tend to have less intense periods with more or less comparably fewer symptoms, but I've been on the pill for years and these days my PMS symptoms tend to be a lot worse but that may just be menopause on the horizon.
Some women are in a lot of pain and nauseous. They want nothing to do with anyone and then really the best thing you can do is just respect that. It ain't personal, she just feels like crap.
Some have pain but no nausea, such as myself. I find I want things to be lowkey. I definitely don't want someone drawing me a bath or anything. I just -- personally -- don't want to be fussed over at all. But that's just me, your lady friend may feel differently.
Other women have very little physical discomfort but may have mood swings. Again, don't take it personally. It probably ain't you. It's just the hormones. For these women, the best thing to do is to back off. Be available if necessary, e. g. don't leave the country

, but if she is angry or confrontational, it may be nothing (or, it may be something. Just 'cause a woman is menstruating does not mean that she has no point or isn't right about something -- it just means that the emotions are running hotter -- so keep that in mind).
Other women get very horny. Others feel very little. Some feel ultra-feminine. Some feel unclean.
Like I said, it depends. I've found that my sense of smell is extremely heightened and things are off, I really don't enjoy anything too smelly, e. g. incense, candles or flowers. Oh, and chocolate. Lots of women -- myself included -- really, really want chocolate. It's sweet, it's an indulgence and it has feel-good stuff in it (the name of which escapes me right now).
So, about the only thing I can advise is, talk to your lady friend and find out how she feels, as opposed to looking for generic signs to divine as if you were looking for signs of a tornado. And offer chocolate.