Ah OK that would make sense. I don't have too much experience with that style of music it seems.
I guess I naively assumed that the OP was referring to songs/pieces written in only 1 key, especially when providing an example such as Titanic. That in combination with the fact they are very much a beginner in music and the Piano, to start explaining about multiple keys, relative and tonic etc etc I think would have been too much to digest.
Addod, you've just run into a common dilemma that plagues both teachers and older students. How much information do you give? Do you give some general rules which are true for simpler music and ignore the fact that music (or any subject) is more complex than that? Do you give an overview and then launch into detail, or do you build things up from one tiny detail to the next? And how, as a student, do you receive that information? There is no single answer.
I didn't have formal instruction until I was an adult. It was on a different instrument. Piano initially was self-taught while I child and then I didn't touch it for decades. I learned certain general principles as I went along, and I could go ahead fast because I had them. But I also based myself on them. When I studied music more thoroughly I discovered some were simplifications and they had distorted some basic concepts that I thought I had understood. I had to revamp what I thought I knew, from the bottom up. When I started teaching theory rudiments and created my first approach for teaching it, I decided not to do the same thing.
Well, the first problem is that you actually want to
teach it - and a student should be
studying. That means you have some kind of organized way of working, where things can come into focus over time. Trying to do that in a few paragraphs in a thread is nuts. When I did teach it, I might say a general concept, but also that there were exceptions, and that this was general "for now". That way hopefully the student isn't going to base himself on something that 5 years later he discovers isn't actually how things always are. The next thing was to develop fundamental things one by one, but knowing that advanced complicated things get built on them later on. It is also how I want to learn. And it's also not easy. I imagine a lot of teachers who have been doing this for 20 - 30 years are still tweaking what they do.
Here:
I think if 50% of the notes were 'accidentals' it's probably in the wrong key ...
As you say, you were thinking of the simpler more mainstream music from the Common Practice period that students tend to get exposed to. But if I'm a novice student and I keep this in mind as a rule, then I might get confused later on. In some music you can get into huge sections that are just peppered with accidentals because of what is going on. It is very unlikely that the OP is going to run into anything like that, so your judgment call was probably correct. I'd say it might be a guideline, but not a rule. It might be better not to worry about music with tons of accidentals, and just learn how to recognize whether the usual music one sees as a beginner is in G major, E minor, C major and similar.
Topic: Why a song can be played in multiple scales?