Was Dred Scott's wife a freed slave?

Yes. Harriet Robinson Scott was freed along with her husband in May, 1857, after they were sold by their owner, Irene Emerson Chaffee, to the son's of Dred Scott's original owner, Peter Blow. Dred Scott had met Harriet while a slave owned by an Army doctor, Dr. John Emerson, in Minnesota Territory in about 1836. After Dr. Emerson died, the Scotts, then in Missouri, asked to be freed from Mrs. Emerson, who refused.
The Scotts sued for their freedom but lost a first trial in 1846. Granted a second trial in 1850 they were granted their freedom by a jury, but the Missouri Supreme Court overturned the ruling. In 1854, Dred Scott appealed the case to the Federal court in Missouri, which became the famous (or infamous) Scott vs Sanford case (John F.A. Sanford of New York was Mrs. Emerson's brother and executor of the Emerson estate). Losing the case again in Missouri, Scott appealed to the US Supreme Court, which in March 1857 handed down a disastrous ruling, known today as the Dred Scott decision: Scott had argued that he (and his wife) were entitled to freedom, having been taken into and lived in free states or territories, but the Supreme Court ruled that as a Negro, Scott could not be a citizen and therefore had no right to sue in the Federal courts.
Shortly afterward, the sons of Peter Blow, Scott's original owner, arranged to purchase the Scott couple from Mrs. Emerson. For reasons that have never been explained, the Blow brothers had paid for much of Scott's legal expenses during his long ordeal in the Federal courts. For $750, Mrs. Emerson transferred the Scotts to the Blow family, who promptly set the couple free.
http://www.thedredscottfoundation.org/dshf/index.php?option=com_content&view=article&id=50&Itemid=55

https://www.historynet.com/dred-scott