Ex-pro football player David Sidoo, 60, will plead guilty Friday in US District Court in Boston to one count of conspiracy to commit wire and mail fraud, a charge that carries a maximum prison term of 20 years, according to his plea deal filed Wednesday. He was a parent involved in the college admissions scandal, in which he allegedly used Singer, the man charged with helping well-off parents get their children into colleges, to have someone take the SAT for his one of his sons and write a college essay for the other.