No, your friend should not disclose deafness on a resume/CV or cover letter unless it will help her get a job. It might be helpful if the job is in a medical field or working with deaf people or similar.
Otherwise, it just gives employers a reason to not interview her. If she can do the job without any accommodation (or if in the US, with reasonable accommodation as per the Americans with Disabilities Act), then she should avoid mentioning it to a company at least until she is offered an interview.
If they want do a phone interview and she can't do that, then she can mention it at that time. The company might choose to do the interview in person or via Skype or other video chat.
If they want to do an in-person interview and no special accommodations are required, then she can either mention it when the interview is confirmed or wait until she arrives. If minor accommodations are required, then she should say so when the interview is confirmed, just as you would do if an interview would include lunch and you have dietary restrictions.
Even if an employer isn't overtly prejudiced, they might feel awkward about interviewing someone who is deaf, thinking they won't know how to act. Avoiding mentioning it early on will allow the employer to evaluate your friend's qualifications without that bias being a factor, even a subconscious one.