I took a look at the installation diagram for the Aprilaire 700M. Are you getting 24 V between the two brown wires? If not then you may need to add the transformer to be the power supply for the 24 V. The humidifier may be drawing too much current and dropping the voltage. This could potential cause the furnace control board to not operate or possibly damage it.

It's very nice of you to help out your neighbor. Keep the phone number of the guy with the truck and tools handy, you may need to use him.

What are some troubleshooting tips to fix my humidifier?

https://www.cooling-heating.net/

The first thing you should do if your humidifier stops working is to check the water level. When the water tank gets too low, or empty there is an automatic shut-off function in most humidifiers that will stop it from working. Simply fill it back up with some filtered water.

Replacement parts for your humidifier should be changed often, such as the filter. If these parts become clogged it will not only reduce your humidifiers efficiency, but also possibly damage the unit itself.

I got it working. As I said, I connected it directly to a 24VAC transformer and it ran just fine, so that eliminated the humidifier. Then I started tracing the wires. Two wires ran up to humidistat and connected directly to the humidifier. At first I thought it picked up power at the thermostat (which is mounted next to the humidistat). Nope. So I rewired it so that the humidifier picks up power off the "humidifier" terminal on the furnace board and it runs just fine.

It simply wasn't getting 24 VAC. The furnace is about 10 years old and to the best of my knowledge the humidifier was installed with the furnace and it's obvious from the condition of the pad that there was water in the unit. I just have no idea how it ran.

Good work! I am glad to hear you were able to help your neighbor.

She a nice lady, who at 81 still mows her own yard. Her husband passed years ago and I know she's on a tight budget so I try to help when I can.