Dear Chenchen Castrourdiales,
You are correct and your Student is not correct. It is not possible to change the quoted sentence --- "She looked at herself in the mirror." --- from the Active Voice into the Passive Voice, for the reason that you have given: "Because we have a reflexive pronoun".
Professor Quirk and others in �A Comprehensive Grammar of the English Language�, write � ... there are a number of exceptions where the active (transitive) and passive voices are not in systemic correspondence� ... ... �Co-reference between a subject and a noun phrase object blocks the passive correspondence. This constraint occurs with (a) reflexive pronouns, (b) reciprocal pronouns, and (c) possessive pronouns when co-referential to the subject�.
In non-technical English, this describes certain conditions when a passive sentence cannot be formed from an active sentence. Among other conditions, it is when we use sentences with (a) reflexive pronouns, (b) reciprocal pronouns, and (c) possessive pronouns which refer back to the subject of the sentence. They give examples of an Active Sentence converted to a Passive Sentence (where possible), while still maintaining correspondence between the two part of the sentence,
The asterisk * at the beginning of a sentence means, �This sentence is Not Acceptable!�
REFLEXIVE PRONOUN
(a)
ACTIVE
John could see Paul in the mirror.
John could see himself in the mirror.
PASSIVE
Paul could be seen in the mirror.
*Himself could be seen in the mirror.
RECIPROCAL PRONOUN
(b)
ACTIVE
We could hardly see each other in the fog.
PASSIVE
*Each other could hardly be seen in the fog.
POSSESSIVE PRONOUN
(c)
ACTIVE
The woman shook my hand.
The woman shook her head.
PASSIVE
My hand was shaken by the woman.
*Her head was shaken by the woman.
The asterisk * at the beginning of a sentence means, �This sentence is Not Acceptable!�
Note: In the example (b) given above, Professor Quirk and others add further information, and give an example.
The Passive of a sentence with a Reciprocal Pronoun can be made acceptable if the pronoun is split into two parts, as follows:
Each could hardly be seen by the other.
To repeat, you are correct and your Student is not correct. It is not possible to change the quoted sentence --- "She looked at herself in the mirror." --- from the Active Voice into the Passive Voice, for the reason that you have given: "Because we have a reflexive pronoun".
I hope that I have helped you.
Les
