That's unconscionable. Why do you feel the need to cover for him to his parents? I was taught "Never shield an infidel from the consequences of their infidelity." I think you should have just said "It's (the woman's first and last name); maybe you should ask your son what it's all about," and let them do the math. If they ask you a direct question, don't lie to them, and give them a direct answer.
You would also be a good opportunity to talk to your daughter about "enabling" behavior. No one should be lending this guy their cellphone to use to call his girlfriend.
