Again, the victim is no longer in charge of the case. They may still influence the disposition to a degree by offering their input as to what they would like to see happen to the defendant, but it is only input and the ultimate decision about prosecuting the case lies with the District Attorney's office and the judge or jury who will ultimately decide guilt or innocence.
Can the victim refuse to testify or say it never happened?
The victim needs to be very careful about changing his or her position. If he or she changes the original story they may find themselves charged with falsifying a police report or perjury. The DA can call the victim as a hostile witness and even ask the judge for a warrant if the witness does not appear when summonsed.
What is the best way to proceed for the defendant if the victim wants the charges dropped?
The charge of Assault & Battery, and especially Domestic Assault & Battery, is a very serious charge. Any criminal charge is serious and will likely effect the defendant for the rest of his or her life. A skilled Criminal Defense Attorney will be able to advise their client on the best way to navigate the dangerous waters surrounding an Assault & Battery charge. I would advise anyone charged with a crime to hire the best Criminal Defense Attorney that they can afford and to then follow their advice. You only have one chance to make the right decision after being charged, but the rest of your life to face the consequences of a poor or hastily made choice.