you really shouldn't use that guy who came out, he is way off the mark.
Let me be clear about something, if you put resilient channel on existing drywall and hang another sheet from that, it will actually perform worse than if you didn't use resilient channel at all.
I know that seems odd, but it is caused by something called Mass-Air-Mass resonance that worsens performance if their is an insufficient gap between the drywall and the next surface. In the case where you sandwhich the channel between two sheets of drywall, there is only 1/2 inch of dead space, not nearly enough.
You are much better of tearing the old drywall off, then putting up channel or sound clips, then a layer of drywall, then a viscoelastic material, then another layer of drywall.
The problem is, contractors don't know this stuff.
John Rice