"Do not play with her."
Translation:उसके साथ मत खेलो।
Not really. It's best to think of के साथ as one single unit or a 'compound postposition' so to say.
In such 'compound postpositions', के is usually used. Eg: के साथ (with), के पास (near), के ऊपर (above), के बीच (amidst) etc.
When साथ is used as a noun (meaning 'company' or 'support'), it is masculine singular so we would use का. Eg: इस चुनाव में मुझे उसका साथ चाहिए ('I need her support in this election').
I speak persian as my official language in Iran, Persian and Hindi have similar grammatic rules. So I knew what such Ugly Sounding senteces in Duolingo means. But I thought its just to be funny. Now I guess they give us such sentences to teach us if you hear such sentences used in India or by Hindi people all over the world(even if they speak English or other languages not fluently), they do not meant to be rude, ugly or whatever. It is Hindi grammer they use in Hindi or other languages they speak now.