Sending text message condolences is permissible if the sender is positive the recipient is comfortable receiving the message in this manner. Some people still consider text messaging condolences insensitive, so it is important to be sure and to take extra care with the wording.
The purpose of a condolence message is to offer support for the other person and to let her know you are thinking about her. Any message must be about the other person and not about what is easier, quicker or less distressing for the sender. Text messaging is a common form of communication and is an acceptable method of communication in difficult situations.