SALEM – The defense lawyer for Donald G. Armstrong, charged with torching a group home in Lynn on Lewis Street in 2008, says his client is incompetent to stand trial.Attorney William F. Martin told Judge Timothy Q. Feeley Tuesday in Salem Superior Court that he has had Armstrong examined by his own medical expert who stated his client is not competent to stand trial.Now it is up to the prosecutor, Christina P. Ronan, to decide if she will contest the report and have her own medical expert examine Armstrong.The judge gave the state until Dec. 20 to decide what option they will undertake as a result of the independent psychological evaluation of Armstrong.Armstrong, 43, a mentally challenged person who is staying at a Bridgewell group home located at 11 Paradise Road in Swampscott, stands charged with arson of a dwelling house in connection with the Nov. 10, 2008 blaze that torched a six-unit fully occupied group home at 28-30 Lewis St. in Lynn.The 4 a.m. fire left 11 developmentally disabled residents and two counselors homeless. Everyone got out safely.The fire was apparently started in a storage area of the basement.Investigators believe Armstrong, who reportedly was drinking, used a lighter to start the fire, which caused approximately $50,000 in damage.Armstrong has been free on his own personal recognizance with stipulations he remain at the group home and comply with all the program’s rules at the facility and is not to leave the facility unless escorted by a staff member.