It is a neurotransmitter made by the brain and is found in animals, plants, fungi etc. It aids in a lot of functions within the body, like mood, sleep and so on.That person's GI's logic for using serotonin to treat UC is probably coming from the fact that about
90% of serotonin in the body is found in the GI tract.
This link helps understand it a little more.
http://www.webmd.com/depression/features/serotonin-9-questions-and-answers