"Unlimited" is just the plan name / description. It doesn't mean you can defy physics. You cannot have more Olive Garden breadsticks than there are atoms in the universe.
Sure, but if you advertise unlimited data and a data rate of 100mbps you should be obligated to deliver 100mbps for as long as the user continues to pay for service.
Except that I guarantee there's some fine print in the terms that says otherwise. Also, cancelling his service means he's not continuing to pay for service.
And, perhaps more importantly, don't be a bitch if once you advertise infinite breadsticks someone tries to take your word for it.