Is it legal for an auto insurer to add new driver onto a policy without consent from either the policy holder or the person being added?

Yes, it may be legal. Insurance protects against risks. That means that it is important to disclose risks to ensure they are covered. If your boyfriend lives at home and/or frequently uses his father's car, it would be appropriate--and may even by necessary, under the terms of the policy--to add the boyfriend as another driver of the vehicle; otherwise, if he is in an accident involving his father's car, it's possible the insurance might be void. Your boyfriend can't be made to pay for another's policy if he doesn't want to; but just as an employer might have to list him as a driver, if he drove for his job, on the employer's insurance, if he uses or may reasonably use his father's car, he may have to be listed there. All that said, for a definitive answer, rather than an explanation of general principals, bring a copy of the father's policy to an attorney, describe the living, etc. situation to the lawyer, and let the lawyer evaluted the specifics of this case.