Can we reduce row lock contention on an index
If this third party app is holding those row locks long enough to cause contention, and

you can't change that behavior,
and the blocked queries are read queries, then

One option would be to enable Read Committed Snapshot Isolation in that database:
ALTER DATABASE [YourDatabaseName] 
SET READ_COMMITTED_SNAPSHOT ON;

This should stop those UPDATE queries from blocking your read queries.
Note that this will affect the results of the read queries, since they will be seeing a point-in-time snapshot of the data being read, rather than being blocked waiting for the update.  It will also increase load on tempdb.  For a detailed treatment of implementing RCSI, check out Kendra Little's post here:
Implementing Snapshot or Read Committed Snapshot Isolation in SQL Server: A Guide