The possession ability is fairly clear that it can work on any creature (outsider or not), any object, or even things that aren't objects like a puff of smoke, a pool of water, the floor beneath someone. The ability even has evil makes creatures easier to posses. If a disease is a valid target I'm not sure why a fiend wouldn't be. If you're houseruling that he cant possess evil outsiders though that's fine, (but let me know) he can just possess one of the items Abduxuel is wearing.