The golden rule to remember is: if in doubt, refer to your pharmacist. Your pharmacist has spent several years training to provide professional advice and guidance. Don't be embarrassed to ask for their help.
If you do refer to your pharmacist, tell the customer what you are doing and why, then go to your pharmacist and tell them what your customer has told you. The pharmacist may want to talk to the customer directly or they may recommend a course of action to follow.