A friend of mine asks, how do you deal with a child who's been left by his father? When should be the best time to tell the truth?

Ideally, I think it's best to hold off until the kid knows how to understand certain circumstances. If the kid keeps on asking the father's whereabouts and is completely aware that he/she has none, then maybe that's the time your friend can explain. However, she must explain it very gently and as much as possible, gradually. Even though the kid cannot fully grasp the entire situation in her mind, she can feel everything--the incompleteness, insecurity and sadness--and this can overwhelm her if things aren't carefully explain. I hope this helps your friend.