Is it permissible to access the internet through a friend's account that is jointly subscribed, knowing that the subscriber has unlimited hours, and would this lead to a financial loss for the institution?

Whoever rents internet service for a month owns the right to access it for the entire month. They may use it themselves or grant access to their friends, provided that the total usage time does not exceed the hours of the month, and that the friends access it in rotation, not simultaneously. This does not affect the company as long as it has agreed to the subscriber's use of the internet for the entire duration.