I didn't tell my bf in the beginning, just kind of let him figure it out from my behavior change. But I don't really recommend that.
I think it's probably best to tell her, then she knows if you're acting strange what's up. Plus, maybe she could be a source of support for you?? In general, I think it's best to be as open and honest as possible with people... So, maybe just tell her you think it became a bit of a problem in your life and you're quitting. If it's a hard day, let her know that you're struggling and why so she won't take any of it personally.