comparative construction used with pronouns, especially 'her'

I think your colleague may be getting at the idea that than is a conjunction, not a preposition. This is a long-running dispute. Those who argue that than is a conjunction, claim that it is followed by a clause in which the verb phrase can be elided. When that happens, the only word left must be the subject of the clause. Since her cannot be a subject, it would be considered wrong. Other people consider than to be a preposition. If that's the case, then they reason, a pronoun following it would be an object, so her would be correct. Both forms are used, but the than she form is formal in register while the than her is neutral.

The fact that the possesive pronoun has the same form as the accusative of the personal pronoun is what seems to be at the origin of this confusion. You also can say: Look at her! or I asked him. These are examples that clearly imply the use of accusative and they're perfectly correct.

This isn't really a matter of logic or grammatical correctness - it's about idiomatic usage.

Grammatically, it's fine to say "I am taller than he", and I suspect this form may even have been more common than either "he is" or "him" a century or two ago. It just sounds a bit dated/stilted/formal today.