Don't be fooled by a health certificate. They are usually only good for a certain period of time and if he were to have a serious illness right away, most require you to return them for an exchange like a pair of shoes.
Unfortunately with poorly bred dogs, it may be years before their "genetic time bombs" start going off. My Lady is a perfect example. She is a rescue was was pronounced perfectly healthy by my vet when I adopted her at age four. Shortly afterwards her genetic time bombs started going off. She now has diabetes, epilepsy, arthritis, allergies, hypothyroidism and now Cushings. All are probably inherited except her Cushings which was caused by taking steriods for her arthritis.
I spend close to $5,000 a year on Lady's medications and vet bills to monitor her conditions. If you are a good friend, don't even take the chance of passing that kind of financial obligation on to her.