Hi Serina, thanks for posting.
I wouldn't think germs would be an issue, unless there is a dog that also uses the yard for potty breaks. The main thing is temperature extremes. Ferrets definitely cannot tolerate heat for any extended period.
Get your buddy a ferret leash and maybe a ferret playpen. Your ferret will enjoy the new environment. How often is up to you. As with any new parent, you can be to overprotective. But KEEP AN EYE on your ferret. Last thing you want is it getting lost.