There is a list of sins that require a vampire to make a humanity roll, like theft, arson, and killing. But I fail to find which humanity dot kidnapping stands on. It sure is an inhumane act to capture someone and keep them locked up. One of my players is trying to cover up a masquerade he caused by kidnapping the mortal instead of killing and I'm not sure if his 7 dot humanity would call for a roll. I'd personally put it on 6 humanity meaning decreasing it to 5 humanity at most. What do you think?
Kidnapping can't be definitively placed on the chart because there are many, many nuances to how terrible it is. A Humanity 10 character need not check for Humanity loss if he's kidnapping a child to save them from their abusive parents, for instance. The reasons for the kidnapping, the treatment and level of free will given to the captive, the captive's willingness to participate, and the duration that the captive is held against their will should all factor in, but there's no hard and fast rule.
I'd suggest to the player that he offer the mortal a job. Kidnapping him for one night to get him alone to talk to him about the Masquerade and why he needs to keep his mouth shut so he doesn't get himself killed shouldn't provoke Humanity loss at that level. Kidnapping him and keeping chained in a basement for a while probably should.