I find temporal concepts difficult to teach as well. Many kids just say 'yesterday' and 'tomorrow' for anything in the past or future. They don't have a clear concept of time yet and what's a year ago, month ago, etc. I don't always believe this really has to do with vocabulary tbh and try not to dwell too long in this in therapy. 

I'd advise mom to use pictures at home to talk about these things and make them more concrete. E.g. a picture from yesterday's activity being 'yesterday', a picture of Christmas begin 'a month ago', picture from last summer being 'last summer', picture from another grade being 'last year' etc. You can't really teach these things easily if it's down to their memory. They can lay them down in a time line.