my friend said the T slur as a short way of saying transgender

my friend said the T slur as a way to say transgender (she already knew it was a slur), because she was talking about her ex, who didn't have a problem being called that by her. she said she only said it because it's a shortened version of transgender

please help me and give me you opinion, because one friend is telling me its ok because shes not using it in a derogatory way but on the other way it hurts me because its not something she should be saying, as we're both in the LGBTQ community.