Question: If a person wakes up needing a major ablution, can he continue fasting until sunset and perform the ablution at evening?
This person’s fast is valid. But as we understand from your question, this person omits to offer his prayers during the day. Ignoring the regular prayers is a great sin. Therefore, one who wakes up needing a major ablution while fasting should have a bath and continue his fast without omitting his prayers.