Though it's not your job to make their dreams come true, but they are friends of yours. If you are the main songwriter, though, they in turn have a responsibility not to be threatened by that fact. Its important for both people to be extremely honest with themselves about it. It will be easier all around if you have thought this over before a situation arises. The fact that you're asking the question is a good sign.
Musicianship makes the world go around.
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