It happens that I am a Sonar user, but I went with a different solution, one that may or may not be an option for the user you are referencing.
My experience was as follows:
In years gone by, I often had trouble with midi notes hanging when using VST libraries within Sonar on the same machine. The problem would specifically occur when making changes to the tempo track within Sonar. At the time I was using a combination of Garritan and EWQL, not VSL. Before then, I had used ye olde Gigastudio, on a PIII slave, and not had that problem (but Gigastudio caused all kinds of other problems). Upon purchasing the VSL SE libraries, it seemed that the best way to avoid that potential issue would be to use a separate slave machine for my samples since that had worked somewhat a few years earlier.
Last fall, I purchased MIR SE and upgraded that slave. My solution has remained to keep MIR on a separate machine from Sonar given the past problems, and so far, that solution has worked. I have some very specific needs that tie me to Sonar (though the new Sonar X1 does not allow what I need to be able to do). Particularly with using MIR, it should be possible to use a fairly modest computer for the Sonar machine, as one would likely be dealing with only two tracks, or perhaps a few more if needing to mix in MIR by section.
Given previous experiences, I do not trust Sonar enough to use VSL (or any VST for that matter) with Sonar in any way other than stand alone on a separate machine. Given personal limitations, I do not have the ability to do detailed research and trouble shooting particularly with a problem I was never able to previously solve, and did not care to chance having again.
Perhaps getting a separate modest computer for Sonar may be a viable solution.