If you cant pass the problem onto someone else, then it sounds like a VFD may be an easy way out. Since your load is a fan, a simple V/Hz one should suffice. Plus that adds 'features' like a logic-level on/off, variable speed, and a ramped start. Obviously the motor is rated at 60 Hz, so you probably shouldnt run it past 60 Hz on the VFD, especially if its not rated for a VFD. I'm no expert in VFDs, but if its non rated to run on a VFD, then reactor on the motor may be required to keep harmonics and parasitic capacitance in the motor from causing issues, but I'd get a second opinion on that.