Hi, I started a Pt 135 helicopter operation less than two years ago and 8 months ago I changed location and aircraft. During the changeover period I was given permission to use the first aircraft for any charter work I had to do whilst my newer aircraft waited for the 135 checks, unfortunately, this did not cut it with the FAA and I had a long discussion with the inspectors about this very subject.
As explained to me, "exclusive use" meant that I had to keep and operate the helicopter for the purpose of my operation, ie. I could not use the first helicopter because I was no longer its operator, even though I had access to it for charter work.
I believe your R44 situation would be accepted by the FAA provided that you had a lease agreement stating that you were leasing the aircraft for more than 6 months(???). If the aircraft owner were to have occasional access, I do not see that as a problem, provided he is not doing Pt 135 work on another certificate.
I hope this helps. Good luck.