Can I recover a SQL database from Standby/Read-Only without a standby file?
Turns out it can be done if you use CONTINUE_AFTER_ERROR

RESTORE DATABASE foo WITH RECOVERY, CONTINUE_AFTER_ERROR


I did still get a warning when I tried it but then did a CHECKDB and received no errors.

RESTORE WITH CONTINUE_AFTER_ERROR was successful but some damage was encountered. Inconsistencies in the database are possible.
RESTORE DATABASE successfully processed 0 pages in 4.180 seconds (0.000 MB/sec).
Msg 3441, Level 17, State 1, Line 13
During startup of warm standby database 'foo' (database ID 46), its standby file ('C:\MSSQL\Backup\foo_standby') was inaccessible to the RESTORE statement. The operating system error was '2(The system cannot find the file specified.)'. Diagnose the operating system error, correct the problem, and retry startup.


Given the warning I'm not 100% certain if I would try it on a production database where I didn't have a good backup.  Given that it's a temp restore it might be worth giving it a shot.