Assuming that your your spouse's best friend has no dependants and is not currently sponsoring another immigrant, she would only need to have income of $18,912.
However, I strongly suggest that you contact a lawyer for a comprehensive assessment, whether myself or one of my colleagues. In addition, you should consider retaining a lawyer to assist you with this process. The benefits far outweigh the costs, especially considering the extra months that you would have to wait if your application were denied the first time around.
If you appreciate the time spent preparing this answer, kindly consider marking it BEST ANSWER or HELPFUL. Also, please be sure to read my disclaimer below. Good luck to you.
Dean P. Murray
The Murray Law Firm
560 Sylvan Avenue
Englewood Cliffs, NJ 07632