Sounds sad-I can understand that when you want to treadmill, you sometimes can only get time to do it when it is inconvenient to your downstairs neighbor- i searched ‘silencing a treadmill’ and this is what i found- ‘one thing that would help would to get the treadmill up onto vibration isolation pads, especially if the mill is on a hardwood floor. You would place a vibration pad on each foot of the machine, or if it does not have feet, you will want to place the pads at the 4 corners of the mill.
Another remedy would be to underlay the mill with a material called SSP floormat. This would roll out under the treadmill, and then your would place the mill atop of this floormat and onto the vibration pads as well. This would be effective at reducing the noise levels transmitted to the floors below’ Alternatively maybe they can put a piece of carpet under the treadmill. There should be a resolution to this problem that doesn’t have to make a machlokes. Good luck in finding it!