As a grad student myself, I wanted a laptop that's reliable (since its my primary computer), quickly serviceable, and lightweight since its going to be carried around a lot. I ended up buying a Lenovo T61 and am extremely happy I did.
The T61 is almost silent, reasonable fast, built like tank, and fairly light. In terms of service, IBM/Lenovo really can't be built. The cooling fan on the laptop started failing a few months ago by making a loud noise. I finally got annoyed with it and contacted Lenovo. I got an option of either sending it in to their depot service (4-5 days) or using a local authorized service provider. On a Friday afternoon I drove down to downtown LA, they took down the information and ordered a few fan (in reality I should've just told them to order it instead of driving down). On Tuesday I got a phone call to bring down the laptop. I brought in on Thursday at 11 AM and got it back around 3PM and everything works great. They have a bunch of locations all over the country (and world probably).
In terms of HP, I knew someone with an HP laptop. Everything was great until the area around the power connected melted due to some issue with the connector (yes, literally melted). In terms of Sony, I had a Vaio from 2001 that had to be sent in to service 6 times over 4 years, each time taking a week. 4 times out of 6 it was due to the power connector breaking. Twice was because one of the RAM modules stopped working (once was due to a faulty Sony BIOS update they later retracted and once was due to the motherboard not working correctly, apparently a known problem that can be fixed by another company for a few hundred).
I will not be buying from either company again. For laptops, reliability and service are definitely the biggest differentiators. Apple is pretty good about and Lenovo is probably the best. The Macbook Pro feels like a very solid computer...the Macbook less so. As for Dell, their computers feel cheap and like they're about to fall apart. If she really is going to be moving it around a lot, there's really no substitute for a thinkpad.