If you are going to be out of the country for just a short time - vacation, for example, a letter granting her decision making authority while you are away should be sufficient. Also, a limited power of attorney including the dates you will be away should be sufficient. A letter should be addressed "to whom it may concern" and should include what decisions she is authorized to make during the actual time you are away. A limited power of attorney should be drafted by an attorney to make sure that the essential things are included. If you are going to be away for a long period of time, please consult an attorney to see what will be best under the circumstances.