I guess it depends how well you know the person of course. I think I'd still find it difficult even if I knew the person very well - and would have to wait for 'the right moment' so hopefully it wouldn't create awkwardness, and definitely while nobody else is there!
At the end of the day Ezzer, if your friend has a solid friendship with the guy, then she could explain it that she is only telling him because she is his friend. There's no way of telling how he'll take it, but even if he does take the huff, he should hopefully soon realise she didn't do it just to be horrible.
I expect if someone told me something like that, I'd be quite mortified and embarrassed, but I wouldn't hold it against the person who told me. But then, I can't totally put myself in his shoes as I know I don't have a problem... I don't think!!