An AV fistula is a connection between vein and artery
to allow access to the as ulnar system for hemodialysis
,which is a procedure that performs the functions of kidneys in people whoose kidneys have failed,like in your mother.
The possible complications of such a surgery are infection,bleeding,swelling of arm,numbness
of fingers etc.
Your doctor has rightly prescribed these anti biotics and these must not be stopped by you of your own.
You should discuss about this with your doctor and let the doctor take his decision.
It is his prerogative to start or stop any medication.