Accept the song and its motivation as it is, and simply comment objectively on how I think THEIR song might be made better.
This is a great way of approaching reviews
I would add that "better" is not "objective" - but purely "subjective" based on your own opinion
Giving your own opinion is absolutely fine (and it really what a review is) - so as long as you keep this in your mind when reviewing you can't go too far wrong
E.g.
Rather than saying "adding a bridge section would make the song better" - you could say "I think adding a bridge might add a bit more contrast to the song to keep the listener engaged"
Giving your own opinion and then supporting it with WHY you have this opinion is (IMHO) much more useful to the original writer
