I'd reassure her that you won't be angry at her if she leaks on her clothes/knickers. I'd also be warning her father to not react negatively if she does that too. I remember I once got my period when I didn't expect to. I was 12 and not really keeping track of mine... and it just came about during a movie. These days I can feel it, but then I couldn't... and got up to a big red patch on my light shorts. My Dad flipped out and acted like I'd just intentionally smeared poo all over hte walls or something. It was horrible. It's not like I had done it on purpose.
Mum used to whinge at me if I leaked on my knickers too. It would make me feel bad, because again, it was never intentional. I sometimes threw undies away, because I'd rather her complain about me losing stuff than make me feel ashamed for leaking.
I'd also let her know to come to you if ever it causes a lot of pain for her. Everyone says periods are painful, so when my kidneys would ache like hell in my teenage years, I thought it was normal. I was in agony, seriously. Horrible feeling. I think it'd have been nice if I knew it was abnormal and could have had it checked out (having a baby fixed it, but jeez, you don't want that happening... lol).