Your gun store person is probably correct.
Penal Code Section 468 makes it a misdemeanor ($1,000 and/or one year in county jail) to possess a "sniperscope", defined as:
"...any attachment, device or similar contrivance designed for or adaptable to use on a firearm which, through the use of a projected infrared light source and electronic telescope, enables the operator thereof to visually determine and locate the presence of objects during the nighttime."
If the IR monocular has an _illuminator_, mounting it on a firearm is a no-no. Magnification is not an issue. If there is no illuminator you may be in a gray area. There are many hand-held illuminated IR monoculars available in any sporting goods store, but you may notice that in California none of them are designed to mount on a firearm. Possessing such a scope and a firearm and a roll of duct tape might put you in jeopardy.
[chainsawkill]
Fish and Game code section 2005 makes it illegal to possess, while hunting, both an IR scope and an IR illuminator. Illegal even if you don't have a firearm. However, the section applies only to hunting for birds, mammals, amphibia, or fish. If you are hunting for velociraptors you can use an IR spotting scope, but you can't mount it on your firearm.
Here's a handy link for finding California laws:
[url]http://www.leginfo.ca.gov/[/url]
Click on the California Law button to go to the search engine. It's simplistic and kind of quirky, but pretty up-to-date and complete.