You got some good answers from jaydalee and Kairi, but here are more details, plus the Whitelist FAQ link.
A provider has to be a TER member to whitelist someone, and she has to have her reviews linked to her username. She doesn't have to pay for a membership. Anyone can be a basic member for free. If she wants full VIP membership then she has to pay. With just one review on TER, she qualifies for Free Limited Provider VIP. She needs this Provider VIP to whitelist someone.
This might be confusing:
A provider can have many reviews here and not be a TER member.
A provider can be a TER member and not have any reviews here.
A provider can be a member and also have reviews, but not have them linked.
Or a provider can be a member and have reviews, and have her reviews linked. In order to whitelist, this must be the case. If she has reviews but is not a member, she has to sign up for membership and pick a username. Then she has to ask for her reviews to get linked to her username. Once this is done she will have Provider VIP which allows her to read her own reviews, use the PM system, and whitelist people. If she has reviews and a membership, but they are not linked, then she just has to ask for them to be linked. It is important that her membership email address is the same as the one on her review profile, as Kairi mentioned.
Here are a couple ways you can tell if her review profile is linked to her username:
1) looking at her profile, is there a button at the top that says "send PM?" This will be along with the buttons for "review this provider," "add to favorites," and "report a problem." If the "send PM" button is missing, she is either not a TER member or does not have her reviews linked to her username.
2) If she posts on any board (discussion boards, ad boards, photo board) there will be a link under her username that says "see my reviews."
-- Modified on 10/2/2016 5:40:46 PM