if(distance < assistant.getLocation() && assistant.isAvailable()){
closetDistance++;
That code just adds 1 to the variable, so it does not tell you anything about who is closest. You need something like:
SET closestDistance to Integer.MAX_VALUE
SET closestAssistant to NULL
FOREACH assistant in Assistants() // check all assistants
DO
IF assistant.isAvailable() // but only the availalble ones
THEN
int distance = location - assistant.getLocation()
if distance < closestDistance // if this is closer than any previous ones
THEN
closestDistance = distance // save the details
closestAssistant = assistant
FI
FI
DONE
At the end of the loop
closestDistance
should contain the distance to the nearest availalble assistant, and
closestAssistant
will be the assistant in question.