Start with this question: Does the owner (student) have a high performance sign off AND is he current?

Yes? then get another FI to give you a sign off in your book (after a check flight of course) so if/when you fly into IMC you can legally log PIC.

No? then you'll need at least 5 hours PIC in that high performance aircraft AND have a sign off BEFORE you can instruct in it. Even though you are only giving Instrument instruction, you can't log PIC in an aircraft that you aren't rated/endorsed to fly, and if the student isn't current, then that's exactly what you'll need to log.