So sorry Jeanie, but I am sitting here laughing out loud about your husband.
At this time of the year, I would not want to mess around with it. You can call the EC and ask...I am sure they would answer this question. OR maybe Heartgard has a number to call??
I had a pup snatch one from another AFTER he had eaten his one month. I called the vet and they told me there was no problem.
When we get pups in rescue we don't know if they have had a Heartgard recently...I am sure at some time or another some have been double dosed. From my understanding it is not a problem; but don't just take my word for that.
Call the above numbers. Ohhhh I know a vet I can message on FB...if he is around and answers, I will message you on FB.