Can you contract HIV virus if someone spits in your mouth?

NO. Though it is a very grotesque experience, you cannot contract HIV virus with someone spitting in your mouth. However, i would make sure that person will never do the same thing to anybody else or even do the act of spitting again :).
Low risk. It is possible to transmit HIV through saliva, especially if it is contaminated with blood and if the recipient has any sores in the mouth. See this site for more info.