Swanage Coastguard posted the photograph of the pair at Old Harry Rocks taken on Friday night.
A spokesman said: "Taken near Old Harry - help us to help you.
"Don’t take risks - this person is stood on a cliff top no wider than 50cms - it’s NOT a path with a drop each side."
A spokesman for neighbouring Kimmeridge Coastguard had earlier issued a plea to stay safe on the coast this weekend.
He said: "With a scorcher of a weekend ahead of us, please be extra careful, go prepared and don't take unnecessary risks when venturing to the coast.