I meant it probably wouldn't damage the equipment if he tried a different mic if they both use the same connector. He can see if he likes the results or not.

I agree without an RTA, TEF, MLS, FFT, etc. there would be no way of knowing how it did for accuracy. Levels and distance it would probably get correct as they are relative, it is the EQ you would have no idea how well it did, only if you like the sound or not. On most receivers you can turn on/off the EQ. While it might not be accurate he can listen and see if it made an improvement to his liking. In this situation it wouldn't be about accuracy.

Being a calibrator I like to adjust these myself as well. I use a TEF system which has an RTA and other measurement types as part of it. I use different types of measurements depending on what I am measuring. For the average home owner which is unlikely to pay someone like me to do calibration it is simply a try it and see if they like it scenario. With a generator and SPL app they could at least verify the basics were correct. I wouldn't recommend anyone use an RTA or other frequency type measurement with a phone or tablet's internal mic to manually adjust EQ.