I agree with Mike and Sparrow. The effect from grapefruit can last longer than 24 hours and some people are more sensitive to it than others. Besides that, interferon is also metabolized by CYP450 and we don't know what inhibiting the enzyme with grapefruit juice would do to the levels of interferon. I would love to find out but not by trying it on a post-transplant patient.
When the risk by far outweighs the possible benefit, then the answer has to be, "Don't do it".