if its really only been a few days then your friend will be fine. even if he's smoked literally all day every day for that period. i did heroin every day for well over a year before getting withdrawals.
any withdrawals will be psychosomatic. though if he's miserable, that as an issue, as that is probably what drove him to do opiates in the first place. benzos are OK short term but long term will make his mental health worse (i actually started using heroin in an attempt to get off benzos). support your friend by putting in time, suggesting fun things to do together, helping with self care if he's struggling to eat or tidy his flat, but don't dole out drugs. they stop helping very quickly.