Theranos founder Elizabeth Holmes, set to be sentenced Friday, should go to prison for 15 years and pay back investors she defrauded, federal prosecutors argued in a blistering sentencing memo. Holmes, pregnant and the mother of a young son, last week asked in her own sentencing memo for a maximum of 18 months — or no prison time at all. She was convicted in January on four felony counts of bilking investors in her now-defunct Palo Alto blood-testing startup out of more than $144 million. Legal experts expect she will receive a multi-year prison sentence. Prosecutors said in their memo that Ho…