The other day, one of the housing employees responsible for the building where I am currently living sent an email notice about painting work in my building. The email consisting of an un-described, un-transcribed image without alt-text where the message is in the image (a la screenshot or PNG or JPG version of a flyer).
I reminded the sender to send a plain text message so everyone can read the announcement. Not only did she immediately thank me for making the observation that she'd sent an inaccessible email, but she wasn't resentful, bitter, annoyed, or angry at being reminded, and then proceeded to immediately resend the notice to the entire building in plain text.
If you want to be an ally, respond the way this person did when called out on abled privilege. Acknowledge your mistake, fix it, and move on.
In a 2011 news article about the torture and murder of a disabled woman by her primary caregiver and the caregiver's mother and stepfather (trigger warning for obvious reasons on that), another professional caregiver for disabled people was quoted as saying, "This woman was tortured to death and the only thing people are saying is sometimes the caregivers get overwhelmed. When did we start sympathizing with the aggressors..."
If you want to be an ally, respond the way this person did when you hear the usual justifications for murders of disabled people.
Allies do not make excuses. Allies recognize reality as it is.