"Matěj wants a rich one!"
Translation:Matěj chce bohatou!
I can't understand why this is in feminine since a rich one in English could be other things, say a friend for example.
Think about it for a second. Let's work with your "friend" example. That could be a male friend or a female friend. The female friend assumption would result in the feminine ending in Czech. Your argument would be equally useful for shooting down the masculine gender in the translation. I mean, he could want a rich wife. Yet only one gender can be provided as an answer, unless we get really tricky and enable multiple correct genders in the "select all correct translations" challenge. (No, we won't.)
So instead, no matter what interpretation of "one" you assume, the exercise in the direction to Czech should accept the adjective endings appropriate for whatever gender you pick, including the one you were after, bohatého. Only one gender serves as the main translation, and the feminine ending is used for that purpose. And yes, the author of the Czech sentence did have a female partner in mind.