Is it true that it is impermissible for someone who has had intercourse with his wife at night to wash the deceased the following day, even after purifying himself from major ritual impurity (janabah)?

What was mentioned is incorrect. Scholars have explicitly stated the permissibility of a person in a state of major ritual impurity (junub) washing the deceased. Therefore, it is even more permissible for someone who is not in a state of major ritual impurity.