I actually had this situation come up while working in an ecig retail store. A young couple came in to buy an ecig for the guy as he was a smoker. His girlfriend then asked me if I thought one would work for her. She had quit smoking 2 years prior and was still using the gum.
I did not recommend it as she had already kicked the most difficult part of the addiction ie, the hand to mouth action, inhale and exhale habit. My concern was that she would re-associate the action with the nicotine addiction. After she thought about it, she agreed.
I did tell her, however, that if she felt she was going to buy a pack of smokes, to come see me and I'd set her up with an e-cig.
If your husband really enjoyed smoking and only quit for health reasons or uses nicotine for medicinal reasons as some people do, then I don't really see any harm in it except that inhaling nothing is still better than inhaling something and if he's kicked that part, I'd think it would be better to stay off it.
Overall, he's a grown man and if he wants to vape, let him vape, but I would talk over some of these points with him and make sure he's certain on his decision first.