Possible crown rot, what went wrong?
 

I have about four orchids, mostly phalaenopsis. My housemate has a few as well. I offered to help repot hers as I had done with a few of my older ones after they lost their flowers. Her larger one had some withered leaves and I suspected it was the substrate. The other orchid was smaller and healthy looking. The two lower leaves had yellowed and fallen off, but I assumed that was due to them being older leaves.

I repotted her orchids. The withered leaves one had some black spots on the roots, slightly rotted, but otherwise decent. The smaller one had glorious looking roots, in fact probably needed a pot upgrade. It was packed in completely in LFS moss. I repotted them both in medium size bark I had soaked, mixed with a small amount of fresh LFS moss and perlite. I informed her to not water them for a week to 10 days.

Well, today, two weeks after the repotting, the small one's whole body fell away from the root system. It still had green leaves, nothing saying it was sick, just today it fell off and looks like it was rotting. I'm devastated. How did this happen? Was it me? Was it leftover affects from being in LFS? It felt strong during the repotting. Can it be saved? Will more leaves grow from the roots despite the leaves and a part of the body falling away?

I'm hoping to understand what happened before I have to explain I may have killed her plant.

what i am seeing would not have been caused by repotting into bark. Either crown rot or southern bligh
I suspect the plant already had an infection going on. Another possibility, cold temperature? and/or watering practices?

I don't think two weeks without watering it would be the cause.

Do you see any granulation ( like pustuals) that are often found with Southern Blight? I agree, you did not cause this, it was brewing before.

I hope you disinfected everything you used.

Yes, in this case I don't think "guilt by association" shows the real culprit. You're innocent; guilty only of being a good roommate who tried. A fungal infection isn't going to zap a plant, it takes a while to develop. Had you not repotted it the same sad fate would still have befallen the orchid. Unfortunately, it happens all too easily to Phals grown in sphagnum and in those nasty plastic "pots" most orchids come in (I'm NOT saying it can't be a successful growing medium, and I have a lot of Phals potted in baskets using sphagnum). Those pots retain water, sphagnum retains water, and it can seem dry on the outer surface when there's still a lot of water in the middle. If the sphagnum is too high on the plant, around the base leaves, it can lead to allowing fungal issues creeping into the stem, especially if the temperature is on the cooler side, and especially if the air around the plant is relatively stagnant. (I could be a sales rep for a fan company, but I'm not.) My friend Bil will possibly pish-posh this idea but when using sphagnum I like to be extra careful to make sure it doesn't trap water around the plant's base.