I did backpacking in Thailand when I finished university (5 years ago) and ended up wearing the same 2 pairs of shorts the entire time. We started in the north with the trekking and all ended up ditching our trainers after as they were wrecked and we didn't want to carry them for the rest of the trip, so I wouldn't go buying expensive walking boots or anything.
Do pack a jumper/fleece if she is planning to stay up in the hills in the north as it can be cold at night, and overnight buses can blast the aircon. Otherwise, although we also went in rainy season, when she is at the islands she won't need anything very warm, and rain tends to be torrential and short lived.
Flip flops will go most places. For the temples in Bangkok I had harem-style pants and a t shirt, but they give out long sarongs to tourists if they are not "appropriately" dressed so I wouldn't overthink it.
I think I did take a pair of heels but ended up being a ridiculous thing to take (unless she will be spending a lot of time in Bangkok where there are fancier hotel bars, but doesn't sound like that will be the case).
The backpacker places are very laid back, and even if you pack a perfect suitcase for her she will end up buying cheap dresses and vest tops when she is there!
Very jealous, hope she has an amazing time!