Delegated Gmail, as suggested by @Bill_Gibson is the absolute best solution for the person who needs access. It is however not a point in time access, as the second person has access until it is removed.
The two alternatives are:
1. Give investigator access to Vault. Very complicated for the investigator, and you run the risk of doing something wrong, rendering the investigator having access to everyone's content.
2. Export the relevant data and giving it to the investigator.
2a. Export with Vault to MBOX and installing MBOX Viewer on the investigator's Windows computer. That viewer is fairly easy to use. When investigation is completed, all files can be deleted.
2b. Export all emails with GYB and inject them into a separate "investigation account" (only used for this purpose), and give the investigator delegated access to that investigation account instead. Delete that account when done.
--
https://wheretofind.me/@NoSubstitute