Selling an iPhone, can the mobile operator block the phone if the bill isn't paid?

[wacattack]

Basically as the title suggests. A friend of mine has had a iPhone bought for her from her boyfriend but just wants to cash it in. However, he has a bit of a reputation for not paying all his bills so she doesn't want to sell it to someone for her boyfriend to not pay the bill and the mobile operator blocks the phone.

The question is, Is it possible for the operator to block a phone when the bill isn't paid even if it is a totally separate sim being used