I second that your girlfriend should DEFINITELY take a lightweight cotton sarong. Useful a a scarf in the cold, headscarf to cover hair in places of worship (especially when I was in Nepal & India, also helps her not to attract too much attention from the not-too-uncommon incredibly inappropriate and pushy Indian men!), shawl for the cold, skirt for the beach, as a quick-drying and surprisingly absorbent towel, blanket for overnight trains when the thick wool ones they give you are entirely too hot for the 40C weather but you want something to cover up with, can be an impromptu shirt/dress, useful to tie things together if you have no string (but bring some anyways. Incredibly useful.) I probably used my $6 sarong more than anything else in my whole trip.
Keep in mind, everything.. absolutely everything... is available to purchase abroad, and with very few exceptions, is actually MUCH cheaper. Nepal is a brilliant place to start out, because of the high trekker traffic there, there's knockoff gear shops EVERYWHERE selling absolutely everything you could ever need to backpack.. fantastic copies of North Face, Colombia, and REI products...and surprisingly good quality for great prices.
Also, the postal systems in these countries (I haven't been to Burma so I can't account) are quite good, if a little slow, so anything you buy or no longer need but don't want to throw away can very inexpensively be mailed home. Even in India, where nothing works how its meant to, all three packages I mailed to various friends back home made it safe. Speaking of India, MAKE SURE YOU ARE STOCKED UP ON TOILETRIES BEFORE YOU GO TO INDIA. You would not believe how complicated it is to locate a simple bottle of shampoo/hand santizer/packet of tissues (which you will need, as everybody wipes their bum with their left hand). And, for your lady's benefit, tampons are absolutely nowhere to be found, perhaps outside of Bangkok.
Enjoy your trip! It sounds wonderful, I'm jealous!