。
She regards him as her teacher.
Literal
She [topic-は] him [object-を] teacher [as-と] is-considering.
考える ('to think, consider') in the Xを Yと考える pattern is the deliberate, reasoned cousin of the more emotional 思う — 'to consider X to be Y' rather than 'to feel X is Y.' 師 is a Sino-Japanese word for one's teacher in a discipline or art (martial arts, calligraphy, traditional music) — heavier than 先生, with overtones of mentor and master. The ~ている here marks a settled standing attitude, not a momentary thought.