I think that's a very personal choice for you but I'm glad to tell you my personal family story to see if it may help in any way.
I was about 14 when my dad met my stepmother, who had an 18month old son. His bio. father had not seen him since he was 9 months old. Sort of the same as you, wouldn't pay support, kept saying he was/wanted to be involved but never showing up, etc. Of course her son started calling my dad "dad", which was okay with me and his other older children because he was just a little kid and didn't know any better. We have always considered him our little brother, whether he's a brother by blood or not.
One day when he was about 5, I had taken him to the park to play, and on the way back over to our house he had said something to me about his "logical" father not being around because he drank too much beer. I said something to my stepmom about it, not knowing if she had told him that or one of her crazy brothers or sisters took it upon themselves to do that, but she said no, it was her, she wanted to make sure he understood that even though my father isn't his biological father, it doesn't make him any less of a dad. His real dad isn't healthy enough to take care of him, and its because he drinks too much. She told him the truth.
Bear in mind that at the time he was 5, his bio. father had yet to ever come see him. Once my little brother turned 6, my dad adopted him legally. His biological father fought about relinquishing his rights in order for the adoption to take place, but all it took was my stepmom telling him he was going to do it or she was going to come after the $8K worth of back support he owed her. She even got him to pay the court fees for the adoption to take place.And my little brother at 6 years old went in front of the judge and when asked if he wanted my dad to adopt him, he said with a huge grin "Yep". They kept him very involved with it.
My little brother is 12 now, almost 13 (there are 4 of us kids, 12 year gap between the he and the youngest of the other three, me:). His biological father has never bothered to make contact with him and we figure it doesn't really matter. My dad will always be his dad. And he will always be our little brother.
I think you can explain it to her, maybe in another year or two, and part of me really thinks the earlier you explain it the easier it is for the child to accept. And then when she gets another year or two older, explain some more. My brother has never really questioned my stepmom about his real father as far as I know. Just not something he really cares about, he has a dad. A great one :)
Like I said, do what you think is best, but know that their capacity to understand and accept is better than what we all generally give them credit for. And good luck with your situation!