This "authority" is in error; the past perfect is not necessarily required here, although there are circumstances in which it would be preferred in formal discourse, and the simple past is not incorrect, although there are circumstances in which its use would be deprecated in formal discourse.
This is in the first instance a matter of context— specifically, what you are trying to express and what time you are talking about, what grammarians call Reference Time.
Perfect constructions do not express events prior to Reference Time; they express a state grounded in prior events which obtains at Reference Time. So if you are talking about a time in the past when your subject's previous unfamiliarity with bears was of immediate relevance, the past perfect will be appropriate:
As she was hiking in the Tongass National Forest she was suddenly confronted by a menacing dark creature her own height and twice her mass. She did not recognize it—she had never seen a bear before she moved to Alaska.
But if your Reference Time, the time you are speaking about, lies before the time at which she moved to Alaska, a past perfect will not be appropriate:
She was in her youth a keen student of wildlife—kangaroos, wallaby, emus—but she never visited zoos, which depressed her, so she never saw a bear before she moved to Alaska.
In less cut-and-dried circumstances, usage varies: formal usage and British colloquial usage tends to employ the past perfect more often than US colloqual usage. If you have an appetite for technical grammar, I modestly recommend our Canonical Post on Perfect Constructions, particularly sections 3.2 and 4. If not, the best 'rule of thumb' respecting use of the past perfect is FumbleFingers‘ Perfect Truism:
”Don’t use the perfect unless you need it.”
In English there are two different voices that may be employed when making a statement: active voice and passive voice.
In the active voice the subject of the sentence is also the agent in the sentence; the subject performs the verb's action on the patient.
In the passive voice the patient becomes the subject of the sentence, getting acted on by the verb. A sentence in the passive voice uses the auxiliary verb be to indicate the tense and the main verb always becomes the past participle.
In your extract, the one is not the agent (the one doing the seeing), but is the patient (the one that is seen), so it is in the passive voice and uses the past participle form of the verb see. I.e. the first extract is the correct one:
.....the one seen at the beginning is ....
Best Answer
Using saw in the second blank implies that you don’t expect to see any further changes in her personality.
I think have seen would be a better option; it matches the first part of the sentence better and assumes there may still be a few personality changes in the months and years ahead.