Firstof all, conglatulations for your father's beginng !! We do 5 second breathing for the first stage
practice( joong ki danboub former part). After about 100 days of steady practice, we teach the second stage(joong ki danboub latter part) with 10 second breathing(inhale, exhale, each).but this is not a absolute rule. We can adjust this rule to practitioners' condition. The point is natural and compotarble and deep-concentrated breathing. Therefore, observe your breathing and compare between the two breathing patterns . You may choose the breathing pattern which is more natural and comportable to you. What is important is not the length of breathing but it's naturalness.
In the case of your father, he is recommened to follow the rythem of 5 seconds for evening his breathing as a beginner. Even and natural breathing is easier to make his seat of danjeon settle.
I have just seen your question today. Sorry for having kept your question cool !