First of all let me say I think it's wonderful you're trying to help her find a balance and as you say, draw the line.

I'm a medium too and my rate is considerably higher compared to what other mediums around here charge. There are a few reasons for this. For starters, you get what you pay for. I'm a natural, I come from a long family (generations) of gifted people, I have been doing this since I was a child, I have spent years studying different crafts, buying books and tools, attending courses and seminars, travelling abroad for long periods of time to be mentored, learn new skills, master a variety of fields...

I value myself and my work. You're not just paying for my time, but for my years of experience and my knowledge. It's also a matter of respecting myself. I would never accept a client who tried to negotiate my rate. I choose who I work with (I don’t accept just anyone willing to pay), and if from the start you’re disrespecting me by not wanting to pay my rates, then I simply have no interest in working with you. I don’t entertain that type of energy around me.

I have a solid base of repeat, loyal clients and to be honest I'm doing remarkably well as it is, I don't need someone to disrespect me and my craft by offering me less than what I charge.
Tell her to stand her ground. The right people will find their way to her. She does not want that type of customers who want her to devalue her work. Seriously, you do not want people like that around you. My advice would be to build a solid reputation, go above and beyond for every single client, do honest work and always have the best interest of the client in mind. If I wasn’t passionate about this, I would go back to my former job. You have to have a passion for helping others, that’s why we do this.

Eventually word will get out and people will make their way to her. I would also recommend never working with people who are not committed. I value my time dearly, and I have no interest in wasting time with someone who’s not committed. So maybe she should consider charging a non-refundable fee upfront.

Another thing is to network with other mediums in her area. Where I live, we all know each other and warn each other of difficult clients to be avoided.