Because it's the biggest, and rubs against footwear, or gets stubbed most often, and is therefore the most prone to infections, which is what this probably is. A quick look by her GP and a course of antibiotics will clear up the problem, but if the nail is ingrowing, it may need a chiropadist's attention, but the GP is the best place to start. These things should not be left, in case 'easily treatable' turns into 'seriously nasty'!
it could be a build up of dead skin under the top of the nail....get her to check under the nail...it can get quite pongy there.....check also in between toes...could be athletes foot....again a pongy situation.
why don't you get in the bath with your girlfriend and give her a romantic scrub...concentrating on her toe! If it still smells afterwards you will know it is an infection of some sort, if the smell goes away then she obviously forgets to wash her big toe!
Well Casey the bath went fine and I think the toe just needed a bit of TLC.
Bottle of wine candles, music it was loverly.. Also i posted a question on here a few weeks ago if sex underwater was dangerous, the replys to that was no, so well you know the rest.. Thanks for the advice Casey