I'm sorry to say that you are absolutely right in that it should have been kept outdoors all winter. They are a marginally more cold sensitive in pots when they're small, but standing it in a sheltered spot over winter would have been fine, especially this year, because we haven't had much of a winter in the UK, really, and you'd still have a healthy plant if you had - they do not do well indoors. In fact, they're listed as H7 on the hardiness scale (downwards of -20C) and in a pot at a young age, hardy down to -15C so long as the soil in the pot doesn't freeze (a rare event in the UK). I don't think hardening it off before leaving outside all the time would have made much difference in this case as its in such a bad state, but hardening off over a week or so is the usual procedure when wanting to transfer plants outside.
It may be too late for the plant - there is nothing you can do other than watch and wait I'm afraid.
UPDATE: As its such a young plant, if its much worse now but still alive, you could try cutting back to see if that encourages some regeneration, though only very small plants might respond to such pruning.