So HOW did you fry it? Reversed positive/negative?
I don't know the Model year of your Mustang, but in general there isn't a relay to the ABS system. For example in the 1999-2004 model year the power is controlled via fuse F2.29 in the Central Junction box (CJB). Fuse F2.29 is powered from the ignition switch.
So if you suspect a "relay" problem and you don't think the system has been damaged, then a new ignition switch would be in order.
Note, the ignition switch and the key cylinder are NOT the same thing. The electrical ignition switch is easy to replace and does NOT require PATS to be re-programmed or a new key.
If this were my car, I would confirm there is +12 volts into fuse F2.29 with the key on. If no voltage into F2.29, then the diagnosis is confirmed.