This one is tricky.
On one hand, a busted fire truck is definitely not AA's fault, and the OP was rebooked out of SXM the next day, which is pretty good, especially for the Caribbean.
On the other hand, AA ultimately decides to serve AXA. If the (lack of) facilities/equipment/services there is causing flights to be canceled then they should pony up to help its passengers, or not serve AXA at all.
If it were me, I'd write a nice letter focusing on the fact that the fire truck problem was known, everything else was canceled, but the AA flight wasn't axed until 35 mins before at which point it was too late to be rebooked the same day. I'd definitely mention your friend's experience a day earlier that had the same canceled flight but were proactively rebooked.
And while the airport AAgents' behavior and original "weather" claim were undoubtedly frustrating, I'd leave that out.