AMS - Meeting another flight prior to international immigration

“ I'm assuming after arrival we get funneled towards security and that I couldn't go over to her arrival gate.”

You won’t go security in Amsterdam after arrival, you will only go thru passport control and customs.
Upon arrival you can very easily go over to her arrival gate and wait for your mother there. After that, you and her can walk together to the passport control area.

You should have a clearly-defined, shared and well-understood plan plan for contingencies: What happens if one of you misses your flight? What happens if one of your inbound flights is delayed, or cancelled? What happens if you can not meet up at the gate as planned (maybe one flight gets pushed to a different/unexpected gate)? What happens if one of you doesn't make it to AMS until the next day?

It's great when things go smoothly. Lots of people have been experiencing something other than smooth travel recently, and there's no end in sight for that. Especially if, as you say, it's been a long time since your mom has flown, you need to have Plan B (and maybe Plan C and D) in your back pocket, and be ready to execute it if needed.

“ What happens if you can not meet up at the gate as planned (maybe one flight gets pushed to a different/unexpected gate)?”

Even with a gate change, planes from the USA will always gate in the Non-Schengen Zone of the airport. So both flights will arrive in the same part of the airport. Arriving passengers can choose to either walk over to the passport control area, or to go to the gate of another flight in the same Non-Schengen part. Or they can choose to go shopping :-)

Perfect, thanks Dutch_traveler! Yeah I meant immigration not security. I'll just tell her to wait outside her arrival gate and meet her there. I'll have plenty of contingencies planned.