Grant a user read only access to someone elses email account

It may be possible to grant another user delegated access to the Inbox and give the user Reviewer (Read only) access.
See https://www.extendoffice.com/documents/outlook/4368-outlook-allow-others-to-send-emails.html

1. At the top of the Mailbox where it reads the user's Name (or email address)right-click Properties>Permissions...add the user you want to have permissions and put a check in Other  "Folder visible".  Leave Permission level at "None"
2. For every other folder you want the user to  have access to right-click >properties >Permissions>add the user....Permission level "Reviewer
    Other - put a check in "Folder Visible"  Apply and OK
3.  For the user who is to access the mailbox
Add the mailbox to the user's Outlook
In outlook File>Account Settings>Account Settings>double-click the user profile>more settings>Advanced tab...add the mailbox to be seen.

I need to do it from the exchange admin center or the exchange management shell … .cant do it from their computer

That's even better
Shell
Add-MailboxPermission "emailaddressofmailboxtoberead" -User "emailaddressofusertoread" -AccessRights ReadPermission