I had always heard that you could eat grapefruit and it had no effect on the medication. You just couldn't drink the juice. I have no idea why the difference.
I talked with someone who has blood pressure problems and they said they knew about this issue with juice and it is mostly about not drinking a lot or at the exact time you take medication.
They figured having a half a grapefruit in the morning would NOT be an issue just as YOU are saying.
You two could be right. I'm not a doctor though, so I can only listen on this topic from others who have dealt more with the issue.