using a join to find data in a second table?
In SQL Server 2016 or better, you can use STRING_SPLIT. I didn't dig in to all of your rules but here is a general approach. Note that the DECLARE @Tables are just there to mock up data. You'd remove those and change @Table1 and @Table2 in the final query to test against your real tables.

DECLARE @Table1 TABLE(folder varchar(32), OwnerNumber varchar(32));
INSERT @Table1 VALUES('folderx','1,2'),('foldery','1,3');

DECLARE @Table2 TABLE(record int, emailaddress varchar(320), samId varchar(32));
INSERT @Table2 VALUES(1,'user1@domainX.com','user1'),
  (2,'Billy@domainy.com','william'),(3,'Admin@domainz.com','Tom');

-- input params
DECLARE @folder varchar(32) = 'folderx', 
        @samID  varchar(32) = 'Tom';

SELECT * FROM @Table1 AS t1
OUTER APPLY STRING_SPLIT(t1.OwnerNumber, ',') AS sp
FULL OUTER JOIN @Table2 AS t2
ON sp.value = t2.record
WHERE t1.folder = @folder
   OR t2.samID = @samID;


Now, of course, with this brittle design, you'll need to protect yourself from values in OwnerNumber like 1,foo,2 but hopefully that's something the home grown application somehow already protects you from.