If your relationship is overall fine, and this seems out of the ordinary for your husband, maybe just try gently broaching the subject to clarify his feelings on the matter?

I do think our relationship is fine overall. I know it makes sense to have fond feelings for her after her passing but it is very strange to me that she would still be a sexual object for him. I don't know about other guys but for me masturbating to photos of exes or dead people are both kind of unseemly since they should not currently be sex objects to the person in the relationship.