The most common and appropriate way to address your friend's husband is by his first name. Using his first name is generally considered polite and friendly, especially if you have a casual or close relationship with your friend.
For example, if your friend's husband's name is John, you can address him as "John" when talking to him or referring to him in conversations with your friend.
Using his first name shows respect while maintaining a friendly and comfortable tone. Of course, if your friend's husband prefers to be called by a different title or honorific, it's always best to follow his preference. Some individuals might prefer to be called "Mr. Lastname" or another title, especially in more formal settings or when there is a significant age difference.
When in doubt, simply ask your friend how her husband prefers to be addressed, and she will likely appreciate your consideration.