Why hybrid batteries go bad
The way hybrid batteries are designed, there made up of many smaller batteries connected together in series to make one big high voltage battery. After about 100K to 200K miles these cells begin to go bad. The big difference in when a battery will go bad, has to do with many variables, for example, the most unexpected reason a hybrid battery will go bad is from sitting for long periods of time (a few weeks to a month in some cases will do it) without being charged (charges while driving). This will cause any old or weak cells to drain their voltage and cause a balancing issue in the pack. This variance in voltage is what causes the "Check Hybrid System" light to come on, or "The Red Triangle of Death".
If you did not leave your car parked for a "long" period of time, and the hybrid battery went bad, this may just be due to the age of the battery and is a sign of hybrid battery deterioration.
In this case, sometimes replacing the bad cell will be fine, but the condition of the other cells will be left to question. There will be no telling when the next weak or old cell may want to die and cause the problem to return.
In the case of battery deterioration, changing one cell is likely NOT going to last, as all the cells are worn down and are having trouble holding a charge.
When a car is sitting over 6 months there is a very high possibility that 5 or more cells have dropped below the minimum allowable voltage and the whole pack will have to be replaced.