The answer is it depends. To make a determination more specific information would be required. What type of FSA is it? How long has the family member been with the Soldier? What is the intent of the Family member with regard to staying in the area? Is the family member authorized in the area? If the Soldier is fraudulently collecting the benefit it needs to be stopped. There are several ways in which the Soldier may still be entitled to collect the benefit if a family member is present. I would encourage you to visit the following links as they can provide clarification based on the specifics of the situation.
MyArmyBenefits has published a fact sheet sheet about Family Separation Allowance for Active Duty Army Soldiers. I encourage you to read the article. If you determine the Soldier is not authorized to receive the benefit, contact your local JAG and/or IG and ask them to review your findings. If the JAG is in agreement with your findings, they can help you prepare a counseling, write a letter of reprimand, or take other action as deemed appropriate. It is also important to determine if the Soldier is willingly attempting to defraud the government.
By arming yourself with the facts and coordinating with JAG/IG you can formulate a plan to approach the Soldier in an intelligent manner. I would not approach them until you have gathered all available facts and determined a potential course of action.