If they both paid the ISP they would both have their own connections. One dude bums off the other so he's got no choice. He can call and get his own line if he doesn't like the rules.
No Holmes, if you read what I posted and understood how renting with someone works you'd understand.
Many bills / utilities only allow one name on the bill.
Let say rent for two is $1500.
Electricity: $125
Water: $40
Phone: $30
CableTV: $150
Internet: $75
Security monitoring: $20
Shared food items: $125
What usually needs to happen is the bill above get divided between the roommates...if they fluctuate then IMHO the best way is to work it out of the rent.
So:
Roommate #1
Rent: $770
Electricity: $125
Water: $40
Phone: $35
Internet: $75
Roommate #2
Rent: $730
Shared food items: $125
CableTV: $150
Security: $20
Now just because Roommate #1 is paying those bills doesn't mean he 'owns' those services, likewise for Roommate #2, nor do they get the right to shut them off after they were agreed too.
This is a big reason why you need to pick your roommates properly...it's so common for someone to meet someone and start staying at the SO's place then say they are paying a reduced rent/bills because they don't 'stay there' anymore....it doesn't work that way.
It sounds though if this is that big an issue they need to figure out a new division of bills and each get their own internet.
You can't make a roommate do shit. If they like to leave shit in the sink and you hate it, you should have screened them better.