This is what you need to do:
1. go to this link - it is the NASA Aviation Safety Reporting System - start here -
https://asrs.arc.nasa.gov/report/cav...formType=cabin
2. file an ASR report which focuses on the fact that you observed behavior by a crew member on AA flight xxx on day xxx at time xxx which clearly showed the crew-member was mentally incapacitated, unfit and not safe to do their job. Keep it at that, neither NASA nor the FAA care about service issues or drinks or whatever - you want to report a mentally incapacitated crew member whose behavior represented a threat to the safety of passengers, and that's it for this report, no further details needed relating to service issues
3. file a report with the DOT basically alleging the same thing - a mentally incapacitated crew member on flight xx/dayxx/timexx whose behavior indicated a lack of ability to do their job
4. contact AA - since you're not the aggrieved party, you're not asking for compensation (right?), but inform them of what happened and also explain you've already filed reports with NASA ASR and the DOT requesting an investigation outside of AA's control that will establish this employee's mental capacity and fitness to fulfill their job duties and you will provide additional context to the FAA if requested
This should be more than enough incentive for AA to take action - in fact they will be very unhappy with the ASR and DOT reports, as it takes the course of action out of their control, and they won't have much of a choice but to address this issue asap