Is it normal to have a change in accent when someone has been in a different country for a long time?

My friend transferred to America and has been there for almost a year now. When she came to visit us she had a very distinct accent whenever she spoke in English. I tried to speak in our mother tounge so it wouldn't be weird for us to listen to someone we knew for over 15 years suddenly speak differently. Is normal though to change your accent when you leave a country?