I wish I could return this plant too, but unfortunately it was not purchased at a store. I bought it from a flower stand at an end-of-summer fair; it was the first time I saw dendrobiums for sale here in Toronto aside from the Internet and orchid shows.
Right now all I can do is either toss it or try to save it... I think I will attempt to save it
I cut all the roots off and applied some rooting hormone on the plant base. It is propped up on its original empty container now with about a centimeter of water in the bottom to increase the humidity. The plant is not touching the water - I am afraid it will cause more rot. I will be looking for a new suitable pot ASAP. I already have a bag of Schultz orchid mix at home that I've been using for my phals, so I will be using it for the dendrobium as well.
As for the light source, my orchids are kept under a skylight (not directly under, but slightly off to the side). The phals seem to like it there - most of them grew new leaves this summer. The pictures are a bit dark because it was raining outside at the time I took them.
Thank you everyone for your advice
I will update in a few months if the plant improves.