If they cant find the letter that IRS sent in January, and they cant retrieve it online somehow, paper filing is the only other option.

https://www.irs.gov/identity-theft-fraud-scams/retrieve-your-ip-pin

Who doesn't remember when they get an envelope with an IRS return address....it's like having a cop's red light behind you

Sometimes that cop gets lost in traffic and doesn't make it into the client's mailbox.  

 Which is scary since anyone could have it if it doesnt make it to the right mailbox, and how does that help reduce ID theft.

Is the client at the same address as shown on the 2021 return?  And maybe there was an issue with the 2021 return, someone filing a fake with a different address.