I'm not entirely sure this question is serious, but what the heck. :)
I've known female classmates in styles like Taekwondo and Kung-fu. They generally said that getting punched in the breast was annoying, because it's a sensitive part of the body. They did not quit because of it.
Many men, when sparring with women, will deliberately avoid punching or kicking to a woman's breasts, but they can't avoid it completely. I can't say if women do the same.
No, they won't get back pain any more than normal. And no, it won't get in the way of punching, though I suppose that does depend on whether their breasts are freakishly big or not. Haha.
I like what Collett89 wrote in the comments on this subject:
Martial arts are meant to extract the most from YOUR body - so if certain techniques are prohibited by your body - you adjust. I have students with hip problems - they don't kick as much as those without - but their hands are much more honed - everyone will have stuff they excel at - and in a fight that is what you use.
I think all of those kinds of worries would be put aside by simply trying a free class or two somewhere. Then they can see immediately whether or not the "girls" are getting in the way of punching or maybe just not playing well with the uniform, etc. All of those kinds of issues have solutions, though.
My advice is to look for schools with a number of women already attending. Then ask them your questions.
And lastly, they should consider getting a chest protector. They're made out of plastic and cover the nipples. It goes underneath clothing.
Hope that helps.