How well she tolerates a drug. For some people, they can tolerate a given drug very well, but another person taking that exact same drug, cannot tolerate it at all. All of our bodies are different from one another, and our bodies respond to drugs differently. The strain of the virus she has makes a big difference in terms of treatment success. Some strains of HIV are sensitive to all of the antiviral drugs.
Some strains are multi- drug resistant.
How long she has been infected. Generally speaking, the longer a person has been infected, the greater the chance that therapy will fail, if treatments are begun late in the disease. This is why we are promoting treatment early on in the disease, rather than later on. What drugs she has taken in the past. If she was on medications for quite a long time, there is a greater chance for drug resistance.