The guy just says it will be hard to determine how it feels because it doesn't leave a footprint. Everyone else in my party got one except him.
This is for Diamond
Dr. Footstep on Route 213 in Sinnoh is a footprint expert, and can evaluate a Pokémon's friendship simply by looking at its tracks. He will hand out a Footprint Ribbon to Pokémon with a high friendship value. Even Pokémon who do not have a footprint can be tested to get the Ribbon. Dr. Footstep has four different ratings on Pokémon friendship. These reflect the amount and size of hearts shown on the Friendship Pokétch application. The quotes vary depending on which Pokémon is being examined. Different Pokémon have different sets of responses, such as Ghost-type Pokémon saying that they are scary and shunned, and inorganic Pokémon such as Magnemite only make the noise "Sssshhnnn". If a Pokémon receives a Level 4 rating, Dr. Footstep will award it a Footprint Ribbon. Bulbapdeia
The reason Drifloon isn't getting a Ribbon appears to be because its happiness is not at the maximum level. Try giving it the Soothe Bell to raise friendship while you battle wild Pokémon and walk around. You can also give it a massage in Veilstone City to boost its happiness.