Apparently he was quite a playboy when he was young.
She's known as a playgirl and always has a new lover.
色事師 refers to a womanizer or seducer, often with a connotation of skill in romance, whereas 発展家 is broader and can apply to any gender.
Compound of 発展 (development, expansion) and 家 (person, specialist). The exact origin is uncertain, but it likely emerged as slang to describe someone who actively 'develops' sexual relationships.