Now unless the song is just a guitar or piano vocal it still needs to be produced. A producer needs to come in, here the vocal and piano or guitar version and make it into something that the masses could appreciate. Now the producer may not be entitled to a writers share or publishing unless negotiated in that direction. That said, sometimes you may be the producer and the writer of the accompaniment to the song. If your friend pays you to produce but does not produce a writers agreement stating the %'s or that the writing is a work for hire then you are entitled to up to 50% of the writers share. Some people pay a producer and believe that everything the producer does is included in the price. Truth is that anything can be negotiated but if a producer, plays, engineers, mixes & masters a song and the negotiation doesn't state that all these thing are included in the price then the friend is responsible for the production cost of the engineer, studio time, mixing....etc. What's more, timing is everything. It is crucial that negotiations for agreements be done AT THE BEGINNING of a project and not at the end or the middle. If in any case a re-negotiation appears or even seems eminent then stop all work until things are sorted out. If you do get caught with work done and %'s not worked out then just delete or Omit everything that the client or lyric writer or top line writer did not create. Protect your talent. We swim in water full of sharks but we like to swim.