Optimally you should not spend any more time with her. You've already admitted that you've fallen for this woman, and it would be healthier for you to stay away from her rather than torture yourself by being around her when you know you cannot have her and the guilt you may feel from having to face her husband knowing how you feel about his wife. Additionally, being around her creates an environment where something could happen. If something does happen, her husband would never believe that this wasn't planned from the beginning by either one or both of you.
Keep in mind that if eventually her husband starts to open himself up to you and trust you, then you may become friends. What is your policy of dating your friends ex girlfriends? If you're like most people, you know its a no go. So what then?
It's time to return the borrowed things, and start putting space between the three of you. You don't have to do it all at one time and you don't really have to give a reason....when they call, just let them know you are bus or make some sort of excuse to get out of it.
On one hand, I want to say that you should tell her why you are backing off, but on the other hand, I think that could potentially open a can of worms and might be best left alone. If she has issues in her marriage, then whenever she decides to split, an opportunity may exist for you. In the meantime, perhaps you can drop them an email now and again, or call and see how they are doing, but maintaining a close friendship with them as a couple in my opinion is not only potentially dangerous but in poor taste as well.
I know you don't mean it that way, and for once I can say that you really sound like you have her best interest at heart. Unfortunately this means you get the short end of the stick emotionally, but you can walk away knowing you did the right thing, and you handled yourself like a real man. I'm here anytime you want to talk.