Had they been able to conceive naturally I wouldn't even be debating this. But it's purely because they did have a choice, that I've got an opinion.
But if they had conceived naturally, they would still have known there was a good chance of the condition being inherited.
IVF gave them the opportunity of screening the embryo, which they declined for whatever reason.
If they had done the screening, they would then have had to decide which embryo(s) to use - implying a decision of discarding the embryo(s) which showed the genetic problem. It is not a case of engineering an embryo to remove the problem as some PPs seem to imagine.
Some people feel able to make the decision to discard embryos with genetic problems (in the same way that some people will terminate a pregnancy where, for example, Downs Syndrome, is detected). However, there are other people who do not feel able to make this decision, or who do not feel that the decision to discard embryos is right.
I feel that the IVF issue in this case is a bit of a red herring.
I would not assume either that, if the IVF was NHS-funded, that screening would automatically have been funded. And if the couple were paying for their treatment themselves, the cost of screening would increase the cost of the cycle greatly, and perhaps prohibitively.
I feel very fortunate that I have not been placed in this position, and would want to think through the issues a lot more before making a judgement.