Sean ‘Diddy’ Combs’ is being slammed by a Grammy-winning friend of his abuse-accuser former girlfriend Cassie. The 54-year-old rapper and business mogul recently settled out of court with his singer ex-lover Cassie, 37, following her filing a suit accusing him of rape and abuse – and in the last few weeks he’s been hit by three other allegations of abuse by separate women. Coming forward to publicly corroborate her long-time pal Cassie’s claims that Diddy was abusive to her, 36-year-old Tiffany Red said in a letter published in Rolling Stone: “I’m breaking my silence, freeing myself from haunt…