Mutual masturbation is a low risk activity, however when body fluid come into contact with a mucous membrane or open wound, it is always a medium-high risk activity. First, don't panic, because you're not sure that the person has HIV. I would suggest that you be tested for HIV and Hepatitis C, as both are commonly contracted during unprotected sex. This will give you peace of mind and help you know your status. I cannot determine whether or not you have HIV, because a test has not been conducted on you. The only way to diagnose is via testing.
P.S. - My advice isn't professional. I do not hold a degree, I have simply educated myself on the subject. If you need professional advice, seek the counsel of a licensed physician.