If they're already doing the interpreting, then concentrating on grammar would, I think, probably not be advisable. I would focus on spoken language. Try downloading some texts and MP3 files of American preachers, especially those from the south, which you could use as texts. I would find out which version of the Bible they are using and get a copy as that will give you a lot of help with the vocab, and you could use texts from it as the basis of lessons.
Concentrate on the pronunciation difficulties they are encountering- try the American Dialect Society for details and information about the dialect, though you should know as you seem to be living in the same area.
Get started by preparing a needs analysis- if they are interpeting, they will be able to communicate, so I would focus on getting that right- get them to tell you what their difficutlies are and find out what situations they communicate in.
I'm noit sure that concise lists of phrasal verbs and idioms would be that much use as interpreters will know many of the basic ones, and those would be the ones on such lists.
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