Is the level of difficulty of the Physics GRE on a par with the Math GRE? I have been speaking to someone who is going to take the Physics GRE, and to get an understanding of what that entails, I wanted to compare it to the one subject GRE I had personal experience with: the Math GRE.

How does the level of difficulty and preparation time required for the Physics GRE compare to the Math GRE?
This is subjective and not a complete answer, but too long for a comment.

I doubled-majored in math and physics as an undergrad, took the physics GRE, and got a PhD in physics 25 years ago. Since then I've mostly been teaching physics, plus a little math, at a community college. In other words, I have a background in both subjects, but my physics is stronger.

I recently participated in an online discussion initiated by a physics student preparing to take the physics GRE. This student felt that studying for the physics GRE would be a big project, because there would be a lot of formulas to memorize and so on. I tried the sample test, and I didn't really feel that it required any significant amount of memorization. There were almost no questions where I felt like I needed some obscure fact that I'd forgotten long ago. I said this, and the student said that of course it was easier for me, since I have a PhD and have been teaching the subject for a long time. There is some truth to that argument, but because I teach at a community college, I actually don't get a frequent workout on any of the material covered in upper-division courses. It really seemed to me that the test did a good job of testing fundamental understanding, and was not something you could cram for or pass by memorizing formulas.

Out of curiosity, I also took the sample math GRE. I think my score was somewhere around the 75th percentile, which was considerably lower than what I'd gotten when I retook the physics GRE. There were some pretty broad areas where I drew a blank, because I hadn't done anything in those areas since I was an undergrad.

Based on all of this, my impression is that the math and physics tests are about equally hard, and both of them do a good job of testing fundamental understanding rather than short-term memorization.