It is not unheard of that a person finds out that a child they thought was theirs, is not actually their biological child. This does not mean that the child is no longer theirs. The child will know the parent as their parent and the love a child has for their parents knows no limit. Often these hurt parents retaliate by abandoning the kids but this means that the child gets hurt in the process.
The child might not be biologically theirs but before they knew that the child was not biologically theirs, did they not grow to love the child? Seeing it from a different perspective, an adopted child is not biologically the adoptive parents but do they love them any less… NO! So essentially there is no difference.
The thing to remember is the child is the one who will be hurt the most by the whole situation.