So, getting passed the initial "ew"-factor, let's face it--she's gonna be dead in like 15-20 years, tops. Or if not dead then at the very least, quite unable to provide any sort of support to the relationship, whether it be money or just the ability to care for the child. That leaves him at 41-46 years old at the most when she's gone, and their "child" will be 15-20 years old, tops as well. Personally, I think it's a bit selfish for them to have a child like this.
They're just setting themselves up for failure. The child won't have much of an involved mother, and all the pressure will be on the father to care for the kid AND his "lover," who won't be able to continue caring for herself all by herself for much longer. It'll be like taking care of two kids, almost.