If that person already knew how to sing properly (and by sing, I'm speaking only of technique - producing a sound, on pitch in a healthy way) I could certainly teach them to make the classical sound in a few hours, yes.
Their musical knowledge would then help them to achieve the kind of sensitivity required for singing the rep in a shorter time than someone coming from no previous knowledge.
I separate style and technique, my students first learn how to sing in a style free manner - larynx balancing, torso and neck support, lower back support, cartilage control, vocal fold configurations, soft palate and tongue use, lips and larynx height for brillance / darkness etc. Then we address style - any given style is just configuration of the elements used in phonation so, for me, classical singing is simply a style - something you put on a voice that knows how to work already.
It can be taught in very little time if the voice is balanced. A good pop singer came to me last month and wanted to learn how to sing in a convincing style for bel canto repertoire. He has had five lessons and can make an extremely convincing bel canto sound.
Of course, now he's realising that in order to sing the stuff in front of people, he has to learn about interpretation, ornamentation, learn all the rep etc... now begins the fun !
So I think you need to rethink your question - I didn't say I could turn Westenra into a classical singer in a few hours, but that I could make her sound perfectly plausible in a few hours with no technique errors. That, however, doesn't make her a classical singer - only a singer who sounds classical. The classical singer requires in depth knowledge of the rep, historical info for interpretation and ornamentation etc.
The accomplished musician you're speaking of certainly knows the rep and interpretation stuff, but lacks Westenra's technique, so unless their voice was also balanced (and by balanced I don't mean classical...) then that would have to be addressed first.
I could, however, (and have, as I pointed out) - take any singer and give them a classical voice in very little time. They would then need to work at becoming a classical singer.