There are no miracles pill or drug or magic to turn his body into an efficient sportsman body while years of abuse.
5 tins of Red Bull before race might kill him. It will surely disqualify him if there is any test before race because there is a limit of caffeine intake before sports events.
All he can do is to take non-caffeinated juice (like orange juice, grape juice whatever he likes) and put few pinches of salt and mix with water. Half water and half juice + salt (one liter water + one liter juice and one tea-spoon of salt). Drink it slowly sip by sip daily until race (he can drink plain water too if he feels thirsty). Drink about 2 liters of this mixture everyday. NOT in one gulp or in one sitting but slowly throughout the day.
Eat whatever abusing food he ate usually. Pizza and burgers are good (if he eats them). Avoid tea, coffee, coke, pepsi, green tea, red bull, chocolate and anything that has caffeine (after race he can resume drinking/eating them).
Before the day of race, on 19 of May, drink about 3 liters (a liter more) of mixture that I mentioned above (or even plain water or just juice). He need to drink more water or water based drink the day before race. Remember, not to drink in one sitting, he need to drink little by little through out the day.
On the race day, do not drink too much water. Just drink normal amount of that mixture, maybe just one liter (again not in go, but sip by sip). He do not want too much urine generating while in race.
30 minutes to 15 minutes before race try to release excess water by urine.
Drink a glass of mixture about 5 minutes before race. Start race slowly and make a pace. Do not try to win, he can't win unless other competitors are heavy hash smokers. Just keep running at comfortable pace, do not increase speed.
Best way is to keep himself on the last in race. Do not try to compete any player. Just reach the end-line at very last. Take rest for the whole day and next day. After race he can eat and drink whatever he used to drink.
Just after the race, about 15 minutes after race, he can smoke to release his anxiety and continue his craving for addiction.