I don't know if she is going to loosely follow Jonathon Ley's route (http://www.phlumf.com/travels/iceland/index.shtml), but that is what my wife and I did this last June/July. If she is, she can read my wife's trail journal for some good info (www.trailjournals.com/teamnasty09). Also, I have lots of map and general information that took forever to accumulate, so I could also help her out there if she is having problems.
That said, I agree that a freestanding tent is more or less mandatory. When it is raining hard for the entire day (or days), the loose saturated soil will not necessarily hold stakes well, even with giant rocks stacked on them. She will also probably want a vestibule to keep super wet/sandy/muddy items out of the tent, as well as add a place to cook. Justin Lichter (www.justinlichter.com), aka Trauma, used a Big Agnes Fly Creek UL1 and said it worked well. Jonathon Ley used a non-freestanding tent and told me that it was just barely acceptable, but he also had pretty good weather for his hike. I used the Big Agnes Copper Spur UL2 and have pretty big reservations about that tent design (you can search for some threads I started about it here on BPL), but it worked for us and withstood the monster Icelandic winds (like 40mph sustained, 75 mph gusts).
I would stress to your friend: freestanding, light as possible, vestibule, wind worthy. Let me know if I can be of further help, especially if she is following Ley's route.
edit: to correct Jonathon Ley's website url.