Re-boiling a soft-boiled egg is fine as long as it has not been sitting out for over 2 hours. If it has been out for over 2 hours, there might be the possibility of bacteria contamination.
It is important to serve cooked eggs immediately because of the possibility of contamination from bacteria like salmonella. Such contamination can cause food poisoning.
Soft-boiled or cooked eggs should be re-boiled within 2 hours of boiling for the first time. To re-boil the eggs after more than 2 hours, refrigerate them first instead of letting them sit out. Then, take them out of the refrigerator and re-boil them, consuming them shortly thereafter.