Not medical advice obviously but grapefruit juice should reduce the effects of codeine by inhibiting the CYP2D6 enzyme that metabolises it into morphine.
On the other hand it should potentiate diazepam, and presumably increase it's duration, by also inhibiting it's metabolisis. Diazepam besides being active itself has active metabolites so it's possible in theory that it would also change the character of longer term effects somewhat although I'm guessing this would not really be noticeable since diazepam itself already has such a long half life that your enzymes would recover by the time the impact of it's active metabolites becomes relevant, and this is especially true if you're already taking diazepam daily.
Taking benzos and opiates together is always going to increase the risk of respiratory depression, although if you're already prescribed both of them - in this specific case the risk is probably minimal, and given that opiates carry the greater danger of respiratory depression I would not think this is especially dangerous. Again though... not medical advice! Probably drinking grapefruit juice with codeine specifically is just going to be a waste of codeine.
