Do not cancel before beginning travel. Your fare may be recalculated as a one-way fare and may actually be higher (crazy, I know).
Since you're on the same PNR: Check in as a group. No-show as an individual.
If they ask your group where you are, have the group say you aren't feeling well.
Done and done.
If the value of the return part of your ticket is worth more than $150, you may want to call AA at about T-30 min before departure and formally cancel so that you can preserve that value minus the change fee.