My wife had one with our 2nd child. We were told "where" the tear was located and to what extent would determine the viability of the pregnancy. A small percentage of tears will heal if they're not significant and no loss of ambiotic(sp) fluids. For us, the tear was significant, and it ended disasterous for us. Within 72 hours of our tear, my wife went into labor because her water eventually broke after days of a slow loss of fluids, and ended up delivering our child at 6 months into the pregnancy.
After, my wife was on heavy antibiotics because infection set in. Our daughter passed away shortly after. I'm surprised the doctor has not put her in the hospital for observation or on bedrest. There is a risk of serious infection and sometimes worse to both mother and baby if the tear causes a slow internal leak of ambiotic fluids due to the tear.
I strongly suggest she get a second opinion, especially if she develops a fever or any other symptoms.