I think what your teacher was talking about is that sometimes speakers of less well-understood dialects will replace certain words with better-known Egyptian Arabic translations, but no they don't straight up start speaking like an Egyptian; that honor goes to standard Arabic if all else fails. Usually though what you see is Standard Arabic (occasionally Egyptian Arabic) words and structures replacing dialectal ones as necessary. Think of a Spanish and an Italian speaking each in their own language but sometimes substituting Latin to make themselves better understood (imagine Latin is still widely taught in Europe in this analogy).