Learn English – Is “He is risen” Correct

Is there anything wrong with mixing contracted with uncontracted phrases in the same sentence?

No, there isn't. You can freely write a word contracting it, and write another one without to contract it. As reported by Mr. Shiny and New, sometimes a word is written without to contract it to put emphasize on it.

As Barrie and Elf said, your intuition is correct. The basis of the error is a subtle one, as with most matters affecting simple past versus present perfect in English, but maybe you can it explain it to the student along the following lines (which you’ll need to adapt to whatever you’ve told them about this tense).

1. The context doesn’t justify an experiential use of the present perfect. These uses make good answers to Have you ever questions (Have you ever swum with dolphins? seen a shark when surfing? eaten snake?). If I ask you, in a deposition, say, Have you ever been aware of the contents of an 43-101 report before reading it?, you could answer: Yes, I've been aware of the information the report contained even before reading it, meaning that at some vague point in the past you had awareness prior to reading. That is crucially different from what your student wants to say: that (s)he was aware at a specific time, just before a particular showing of a film.
2. Nor is a resultant state perfect (present perfect of result) appropriate. When these combine with predicates that denote nonpunctual events (know the answer, believe the rumour, study French; as opposed to punctual burst the balloon, reach the summit), they usually mark the onset of the attained state (I’ve known the answer for five minutes / since you walked in the door), or indicate overlapping intervals (I’ve known the answer for the whole time you’ve been talking). They are infelicitous with phrases the mark other time points (e.g., the endpoint I’ve known the answer until you walked in, or a midpoint, Having figured it out five minutes ago, I’ve known the answer when you started explaining). The problem with the student’s sentence is that, in I have been aware before watching, the before clause highlights that awareness preceded watching, so an overlap reading is inappropriate and an onset reading is impossible.

The basic idea is, run through the list of uses you’ve given your students for present perfect and explain why none is apt here.