She is providing a professional service and, I'm guessing hasn't lowered her price. You should expect the same quality as in the past. Since she is a friend, it's a sensitive issue, but you should bring it up, especially since it's been the last several quilts. If it's the machine, she may not have noticed. If she's just getting burned out, I don't think it's fair for her to continue to do sub standard work. Maybe it's time for her to end this particular service or train someone to take over.