I've a master and a PhD in physics and I've been working as a phisicist during the last 5 years, this is my insight.
First of all, not every kind of phisicist does software development, if you don't any laptop and even a chromebook, would do. However, this is getting increasingly rare, only really outstanding scientists can afford this luxury, so chances are that she'll need a real laptop of some kind
Windows, Mac and Linux all have some advantages and some disadvantages, here are the nost important:
Windows: you have all the graphical software you need, whatever field you are going to study. Some communities rely on specific commercial software which are typically only available for Windows and Mac. On the other hand, developement on windows is going to be difficult: all the developers use Mac or Linux for a reason, just installing python on Windows is a pain, let alone using makefiles or similar.
Mac: it is a good tradeoff, you have almost every graphical software and a developement environment which is relatively well supported. Between ports and fink and homebrew, installing developement software on a Mac is always on the hedge of becoming a mess, but not as bad as windows. On the other hand, you'll have to spend big cash on it. Not just for the hardware, also software tend to be more expensive, i.e. the Intel compiler suite is free for academic use on Linux and Windows, but for Mac you only get a reduced version (and it used to be 150$ until last year!).
Linux: by far the most powerful development environment, ad everything is pre-packaged and tidy, you waste no time installing packages and fixing dependencies like on mac. You trade off by not being able to use some specific proprietary softwares, popular in some communities; it is better to keep a windows partition just in case. You may need to do some tweaking in order to get it to work properly on your laptop, and battery performance may never be on par with the same laptop on Windows (or MacOSX).
Personally, I do a lot of developement. I would never use anything else than Linux on the desktop, but I'm sort of tempted to go for a Mac for my next laptop in a couple of years. My main problemd on the Linux laptop (a 2nd gen XPS13) is that skype for linux sucks. Or maybe in a couple of years I won't have the need to buy a laptop anymore, I'll just buy a tablet and take out my old laptop those few times I need to ssh from home. When I was a student, I used a cheap (still 1k$ at the time!) HP laptop and dual boot it with Linux and Windows, I could write my PhD thesis on it no problem, I liked that it was quite bulky with a big keyboard, suspend to ram never worked properly.