A healthy diet will help recovery from any illness, and benzodiazepine withdrawal is no exception. The best advice seems to be to maintain a balanced diet, because it is likely to have all the components that the body requires, and not to be obsessive about it. Withdrawal produces enough anxieties without adding to them.
However, problems can arise in the stomach and intestines because of the general stress caused by withdrawal. So people suffer nausea, constipation, indigestion, diarrhoea and so on, most of which can arise under normal circumstances, and are susceptible to conventional treatment. So if, for instance, someone has an attack of diarrhoea caused by an intestinal virus, the normal treatment of keeping warm, drinking plenty of water, and replacing the lost salts would apply. If on the other hand, the problem is stomach ache caused by fear of some event, there is probably nothing the carer can do except be reassuring and sympathetic.
Some foods should be avoided, or consumed sparingly, principally stimulants such as tea, coffee and alcohol. A cup of black Java coffee just before bedtime would not be good idea for someone who has difficulty sleeping. It is probably enough to reduce the intake of stimulants rather than eliminate them, but my wife decided it was easier just to cut out wine altogether. There is some evidence that reducing the intake of sugar is beneficial, but that is probably a good idea in any event. My dentist would agree.