I must admit this is one of the more unusual posts we've had in a while.
When I inquired about the tree the clerk at the store told me that it had been sitting there for roughly 3 months with no one ever so much as looking at it
At first I thought you meant that it had been totally neglected for three months but that would surely not be possible as it would be long dead by now. Someone at the store must have paid at least a little attention to it.
I noticed that the previous owner used a hard foam sponge rather than actual soil.
Next, I have never heard of a live plant being grown in florists foam, which is what I assume you mean by hard foam. I have to wonder if it is even rooted at all. A Juniper can appear alive for some time after it has expired, or has been cut. Could it be that this is not actually a live plant but a large cutting, so to speak, that has been secured in a pot but is not actually potted?
Are the rocks glued in place? This is another trick used to secure poorly rooted material temporarily until it becomes established or dies, whichever comes first.
I don't mean to sound pessimistic and I hope I'm wrong as it looks to be better material than is usually found at retailers. Are you aware that Junipers should not be kept inside? Many have tried with the vast majority failing. So that's the first thing I would suggest, keep it outside in a spot that does not receive direct sun, except perhaps in the morning. Mist it frequently to help check the needles from becoming dehydrated an more than they already have.
Re-potting will definitely be in it's future but I hesitate to suggest that since you are a novice and the tree is obviously in distress. On the other hand the foam situation is disconcerting and not appropriate at all.
This one is a real puzzler. Does anyone else have any thoughts?