The Todd Parr books are brilliant - my ds's love them.
It's hard to give advice without knowing the exact situation of your friend. Our ds's know that they've got a Dad but that he doesn't live with them. But he's a friend of ours and sees them regularly, so it's probably a bit different.
There's no point denying to any child (in my opinion) that they were 'made' by a woman and a man - their Mum and Dad - the difference comes in who lives with them/who they see/who brings them up. So ds's know they've got 2 Mummies, a Dad who doesn't live with them, 2 grandmas and 3 grandads!
If we'd have used a sperm bank we probably would have gone down the line that a man helped us make them - how kind and wonderful that is - but that he doesn't help bring them up. Or something.
I suppose I'm trying to say all children need some kind of explanation, suitable for their age, but not a flat out denial of how they got here. It's really tricky but having just had the yoghurt pot and syringe talk with ds1 (nearly 7 and through a logical line of questioning got there himself!) I know how difficult it is!
Dp and I have also met with ds's teachers each time they've moved up a year, to go over our approach to talking about our family - things like ds's might want to/choose to include their Dad in family drawings - and both Mums etc. In fact ds1's special person he talked about in his class assembly was his Dad These Dads who get all the credit, hey?!