Both of these sentences are grammatically correct, but they carry slightly different meanings! 'Don't praise him to me' means 'Do not tell me how great he is or express admiration for him.' This perfectly matches your context of 'I know what kind of person he is,' because you are telling the listener that you already know his true character. 'Don't brag about him to me' means the listener is boastfully showing off about their own relationship or connection to him. While both are correct, 'praise' is the most natural and accurate choice for the situation you described. Excellent question, and keep up the great work with your English studies!

'Don't praise him to me' sounds better.