These people know exactly what they're doing. They're trying to get free services and supplies, at the expense of your "friendship".
Next time they ask, I'd give them your husband's business card and suggest they schedule an appointment for a formal consultation or services.
If necessary, strengthen your point by saying he doesn't do free projects because of liability, and he'd prefer that all of his equipment and time is used under a legal contract for insurance reasons. Say "It's really the best way for both you and him."
This is neither rude or unfair. It's just honest business. You are right, they are using his work equipment. What if they bust it up or get hurt using it? I think I'd just put it on the table and tell them they can't just use his stuff anymore because he needs it for his job. I guarantee you, if you put their next request on a business level, they'll either pay up because they honestly need the services or stop hinting for "free" services or asking to borrow things because no one can argue with the truth.