Ick. For the mental health of that child, it needs to be raised by someone else, and preferably never have to know its parentage, although I guess that isn't possible since the kid will have to be warned of potential health problems.
Socially unacceptable as it may be, genetics is pretty much a 'Garbage in, garbage out' proposition; unless the father is carrying some extraordinarily bad genetic freight, the odds of it having any unusual genetic effect in the first generation of this situation are very remote.
Successive generations of it, that's another story entirely. Like with stock animals, it's 'successful line breeding' for a generation plus when you start with sound genetics, but 'excessive inbreeding' when the bad recessives ultimately get too reinforced.