How do I map a SQL Server login to a user that already exists in a read-only database?

Question

One of our database users complains that they can't select from one of the databases in this particualr instance while they have no problems in accessing others in this very same instance. The one that they have access problems is shown as "Standby / Read-Only". The others are just normal.

So, I checked the Security -> Logins for the given principal and looked at the principal's properties to see if there were any user mappings to the read-only database for this login. There were no such mappings for that particular db while there were mappings for other databases and all with "db_datareader" role membership. Initially, I wanted to add a user mapping for this db as well; However, it failed with the below message:

"Failed to update database X because the database is read-only"

Then, I checked the read-only database and found that a user exists in there with the same name as the account that had this issue. It also had "db_datareader" role membership. So, I guess I just need to link the two; however, I don't know how. Please help me! Thanks.