An abscessed tooth is an infected tooth. If it is in the upper jaw, it can cause all sorts of problems...sinus infections (as tooth roots reach up into the bone that makes the sinus cavity), a fever, headaches, etc. Tooth infections are very bad because they can lead to loss of more teeth and bone. In some cases, untreated infections can get to the brain and can actually kill. Not a good thing. An abscessed tooth will be VERY painful and will be very sore to the touch. However, abscessed teeth don't stay infected for too long. They will rot and either fall out or have to be pulled. If the symptoms your friend is displaying have been observable for more than a few days or so, the bloodshot eyes you are speaking of are probably from smoking to much weed or the result of drinking too much.