My cousin revealed to me she sleeps around with guys sometimes. Should I inform her mother (she is 24)?

We are both 24. Since she is an adult I figure I have no obligation to tell her mother. Her mother can be nasty and plays favorites among her children. However she would have the right here to be angry. My cousin said she sleeps with guys who propose sex as "DTF" which is Down to (F word). To me that... show more We are both 24. Since she is an adult I figure I have no obligation to tell her mother. Her mother can be nasty and plays favorites among her children. However she would have the right here to be angry. My cousin said she sleeps with guys who propose sex as "DTF" which is Down to (F word). To me that sounded very vulgar and I told her that. She said she has not done it since last winter.

So do I need to tell her mother or am I best just keeping my mouth shut as I told the cousin it sounds grouse. I am not terribly close to this cousin as our mothers don't like each other but I do see her 3 or 4 times a year.
Update: Sorry, I meant to say gross, not grouse.
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