Music theory is generally based on interval, harmony, mode and tonality. If your friend is good at solfeggio, there should be no problem. Generally speaking, it is a big problem for people to practice solfeggio and ear training. If you have knowledge of music theory, you just need to understand it and do more exercises. Bookstores usually buy some exam materials from previous provinces. Please hire a music theory teacher if you can, or you don't have to take one-on-one music theory class, which is more expensive. You should find a training class and take a big class.