According the research I have done (and I am by no means a doctor), it appears that your friend may have canker sores or nasal congestion.
Often canker sores will ooze yellowish fluid onto the tongue, which might explain the yellow colored tongue. It also causes dry mouth.
Another idea may be nasal congestion. If your friend is stuffed up in the nose, he/she may be breathing through his/her mouth which causes dryness and allows for a yellowish bacteria to grow on the tongue. The website I have linked below suggests a homemade mouthwash to help the problem. Follow the link to see how it is mixed.
However, I would highly suggest that your friend see a doctor, because only a qualified physician can diagnose what is really going on by seeing you and recommending a treatment specifically for your ailment.