Joan Parker, the philanthropist and widow of mystery writer Robert B. Parker, has died. Parker, a longtime Cambridge resident, died Tuesday, according to Helen Brann, a longtime friend and agent to Robert B. Parker. Joan Parker had been diagnosed with inoperable lung cancer in August 2011, and was receiving treatment. A tireless fundraiser for a host of different charities, Parker was barely slowed by her illness. Last month, she co-chaired the annual fund-raiser of PFLAG, a national nonprofit supporting parents, families, and friends of lesbians and gays. (Parker’s two children, Dan and David, are both gay.) Robert B. Parker, best known for his “Spenser’’ detective novels, died in 2010.