I have an AED and am trained in its use.
To start, owning an AED is only one component of helping someone with heart problem as a first responder. If you don't know CPR, the chances of bringing someone around with the AED alone go down exponentially.
An AED will only work in cases of V-fib or V-tach. If either of these conditions are detected, it will advise you to defibrillate. If the patient is not actively in V-fib or V-tach, it will announce, "shock not advised" and you will not even be able to administer defibrillation.
To the OP, having an AED is great "insurance" just like having an extra fire extinguisher handy. In any case, whether you choose to purchase an AED or not, take a CPR course and also consult with your physician regarding your husbands condition and what you can best do should he have a problem.
Do note that once you own an AED, it is good for about 4 years before requiring recertification.