Word of mouth recommendations are actually great. Ask your friend what she thinks of the musician, what she has heard, and if she has first hand experience of the kind of job that he does. If you are still interested, call the musician and have a meeting. Ask him to bring any samples of his work that he might have. After meeting the musician, and discussing your wants with him, you should have a good idea of whether or not you would like to hire him. Also do a little searching and compare what your friends friend is offering you, to what other musicians might be able to offer you. Just remember, if you do hire your friend's friend the agreement is between you and the musician. Your friend is not a part of it. And, hopefully everything will go well and there will be nothing to worry about.