Here is the passage:
Marina, the beautiful mermaid, wanted some
tuna salad. But had a small problem since she was allergic to celery. At
Sammy’s Sub Shop, Marina hoped to find tuna salad free of this dangerous
vegetable. Flopping across the tiled floor to the counter. Marina placed
her order and then checked her sandwich for celery. Not noticing, however,
the spoiled mayonnaise. At five o'clock that evening, Marina became
violently ill with food poisoning. When a lifeguard
at the beach discovered the problem, he called 911.
Even though the mermaid had fishy breath. A handsome paramedic gave her
mouth-to-mouth resuscitation. Wailing like a sick dog, the ambulance sped
off to the hospital. Where the doctor on call refused to treat a sea creature
with a scaly tail. A kind nurse, however, had more sympathy. When this caregiver
returned with a liquid antacid. Marina drank the entire bottle, feeling
an immediate improvement. The mermaid told the rude doctor never to swim
in the ocean. For she would order hungry sharks to bite off the doctor's
legs. While sharp-clawed crabs plucked out his eyes. Tossing her long hair,
Marina thanked the nurse for the antacid. And took a mint from David,
the handsome paramedic.
A complete sentence must have a main clause. A main clause will follow this pattern:
Subject + Verb = Complete Thought.
In the highlighted item, he is the subject, and called is the verb. This pair makes a complete thought, so you do have a sentence.
Lifeguard and discovered are another subject-verb pair, but notice that the subordinate conjunction when precedes them, making the first part, when a lifeguard at the beach discovered the problem, a subordinate clause.