Upping the Stakes.

Excellent question, and honestly, there's no right answer. You're going to get entertaining results either way. My personal recommendation would be for the druid's ally to have some knowledge about the creature, and to say something along the lines of, "The nothic is indeeed a dangerous creature, but alas, my duties keep me here. However, I know what you are capable of, and this is a problem I think you and your allies can handle." If they already spoke to the druid and you portrayed him as not knowledgeable about it, he could have done some research with other allies to bring this information to the party. By way of additional aid, maybe he sends them along with a healing potion (if you're feeling generous).

I generally don't change monster stats in response to developments in the campaign, but as you said, they're expecting a nightmare fight and this is one of those opportunities where I might like to meet their expectations. The extent to which I would buff the nothic depends on three main factors.

1) Whether the druid ally accompanies them
2) How much of the hideout they cleared out, i.e. whether the nothic is the only fight for that adventuring day, indicating how much of their daily resources they can afford to bring to bear on it
3) Whether the party has gained any other notable resources (levels, equipment) since they last met the nothic

Again though, it could be just as entertaining to leave the nothic as is, and measure their surprise when they drop it handily. I don't think you can go wrong, whatever your final decision.