Hi Jeff,
As a Brit who has lived in Mexico for the best part of a decade, I remember well how your father must feel. I now do public speaking in Spanish. I do not sound Mexican but everybody understands my accent and nobody is quite sure where I come from.
If his accent is that much of a problem, the solutions are relatively easy to get him to the stage of being understood. He is basically reading Spanish words in English but words that look like they are in English are pronounced very differently. I hear it all the time in the local gringo population.
Actually, since this is one of the US retirement hotspots, who knows, I could even be doing the lessons in person!