What is the earliest point a passenger arriving from outside of the EU and from the UK can meet up at Schiphol (Amsterdam) airport?

I believe that since the central security changes last year, Willeke's answer is most likely incorrect for you (although as I've not been on an inbound flight from an "insecure" country since June 2015, when this all changed, I am happy to be corrected by someone with first-hand experience).

Schiphol airport describes the changes in detail, but in summary:

• Departing passengers are now screened before entering the departure lounges at all, immediately after checkin. Similar to most airports.
• Arriving passengers from "clean" non-Schengen countries (such as the UK and US), that meet EU security standards, are deposited directly into this shared departure area. This hasn't changed, but it does now make transfers easier for them, no need to re-clear security at the gate. They leave by heading towards passport control.
• Arriving passengers from countries that don't meet EU security standards, and I strongly suspect this includes China, are shepherded along separate corridors on a separate level directly to passport control, and avoiding the clean passengers.

So - your partner should arrive into the main departure area. You should be herded directly to passport control. You could meet up before passport control therefore, or at the baggage claim. It's unlikely that you'll be able to get into the clean departure area without a boarding pass - although you could try, perhaps they won't check. It's unlikely your partner will be easily able to make it backwards along the unclean arriving passenger route - although again, that may be possible.