The form that is being sent to the customer is public and the customer doesn't need to register or log into Salesforce to update it. Hence, we need to provide access.
P.S. Discover Update Client Details, the easy way to keep client information up to date in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsATUAZ