Power of attorney would be best choice. The document can be sent by by computer to be witnessed and notarized in foreign country and sent back. Communicate with landlord ahead of time to see if they will accept a copy of the document-if not-arrange for it to be "air mailed'.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
It's entirely up to the landlord, really, if he/she is willing to accept a simple letter signed by your friend authorizing you (by name) to act in her behalf regarding all her property within the premises. Your friend could even call the landlord and speak personally to him/her, identifying you and confirming the arrangement without the necessity of a written document. A POA is fine, but seems excessive. It could be sent as a pdf through e-mail, and then signed, scanned and re-e-mailed back to you, but if the landlord is satisfied with the your friend's say-so, then that's all you need.