The appropriate and respectful way to refer to your friend's wife is by her first name, unless she has indicated a preference for a different form of address. Using her first name shows familiarity and is generally considered polite.
If you feel that using her first name might be too informal, especially if you are meeting her for the first time or in a more formal setting, you can add a title before her name. For instance, you can address her as "Mrs. [Last Name]" or "Ms. [Last Name]" followed by her first name. Make sure to use the correct title based on her marital status and her personal preferences.
It's always a good idea to be attentive to how your friend introduces his wife or any cues he may provide about how she prefers to be addressed. When in doubt, you can simply ask your friend how his wife prefers to be addressed to ensure you use the appropriate form of address. Showing respect and consideration in how you address others is an essential part of maintaining good relationships.