You will find him in Newland Desert at 2800 x 1680.
If you speak with him regarding an antidote, and ask whether he can help you, he will ask that in order to produce an antidote he would need a toxin sample to find out what could possibly cure it.
You will receive a mission to gain a Toxin Sample from the aliens to give to the herbalist. You will find these samples drop on occasion from ground and ship aliens from your city raids. We seem to have found a higher drop rate on lower title level raids than Title level 7 alien raids.
Armed with your toxin sample, you can speak again with the Herbalist and tell him you have the toxin. When he runs an analysis on the sample he states that he has most of the ingredients required to make the antidote, although those will cost you 40,000 credits. Unfortunately he is missing 1 component :
Your mission will update: Find Restorative Herbs for the Antidote that herbalist Gerard will try to make for you. The mission text states that to find these herbs, you need to find the Rhinomen. Fortunately we are right in one of their camps!
You have a 10 hour timer on this mission.
The herbs are NODROP Unique, but we managed to find some very quickly on the Rhinomen close by:
On return to the herbalist, tell him that you found some herbs and show them to him. At this point he will ask you for the herbs and the 40,000 credits.