In that course create one activity with completion tracking set such that completion of that activity they are done ... thus can get the cert.
Could be a quiz ... 1 question - Do you want your cert? (answers yes or no). Yes or No response and they are done made 100% on the quiz and thus can see the link to the certificate to download.
Truth in sharing ... never done above! Kinda screwball, but .... worth trying?
'SoS', Ken