I saw that The Netherlands has a COVID vaccination requirement for "transit/transfer"? I'm assuming this does not apply to an airport transit (e.g. stay in terminal)?
That is not how I would interpret that language (actually, I would assume it means the opposite of what you are assuming).
Look here: Checklist for transit or short stay in the Netherlands (web page from Netherlands government)
It says (in part):
European entry ban is in effect for some people, barring them from travelling from countries and areas outside the European Union or the Schengen area to the Netherlands and/or the EU/Schengen area.
You may transit in the Netherlands only if one of the following applies to you:
You are travelling from a safe country/region where the risk of contracting COVID-19 is low.
Note: It's a short list, and the USA is NOT on it.
You have been fully vaccinated.
The purpose of your trip falls under one of the exemption categories:
You are travelling through the Netherlands to a non-EU/Schengen country...(more details)
Poland is both an EU and Schengen country, so this does not apply.
You must be able to show you have permission to enter the EU country/Schengen country in question. You can do this with a note verbale from that country’s embassy, for example, or another type of document that proves you can travel there. If you do not have such a document, the Dutch border authorities will determine whether you may travel via the Netherlands. Without a document showing you have permission to enter the EU country/Schengen country in question, you are advised to fly there directly and not via the Netherlands. If you will be making a short stop at a Dutch airport you may need an airport transit visa...
It's possible this web page may be out of date, or in practice that requirement may not be strictly enforced (see the post above) -- things do change often -- but, the way I read this, for those not vaccinated, it may not be a simple matter of breezing through the airport and going on their way. I could be wrong, but if it were me, I would want some clarity from an official source.
Best advice: You should check with the airline for starters (I'm hoping/assuming your relative has a single ticket booked all the way through to Warsaw; if not, if it's on separate tickets, that could further complicate things, as then they might need to 'officially' enter the Netherlands to make their connecting flight).
If your relative is vaccinated but doesn't have documentation, they should find/get documentation to support that (it should not be difficult). If they are not vaccinated...travel is yet another good reason to welcome the jab (assuming they can). FWIW, new omicron-specific vaccine boosters are coming in just a few days...
Hope that helps. Good luck.