ATM transactions really aren't open to dispute, they either got the money from the machine or they didn't.
If the dispute involves a point of sale (POS) transaction, then VISA does place some obligation on the issuer to assist the cardholder. If we're talking about merchandise not received, defective merchadise, services not performed, etc. then the customer does not have Reg E coverage. This means that you are permitted to require that the customer first try and resolve the dispute with the merchant. However, if this is not successful, and the customer's sitation is covered by a valid chargeback reason, you are contractually obligated to assist.
When you file a chargeback on the cardholder's behalf, you must provisionally credit their account at that time. Since Reg E does not apply, there are no investigation timeframe limitations. You can work through the chargeback/arbitration process for as long as is needed to resolve the dispute and can revoke the credit in the event VISA sides with the merchant.
I can do all things through Him who gives me strength. (Phillipians 4:13)