Can my roommate really use a collection agency to get a portion of a bill?

Roommate A took the responsibility of paying for gas. Roommates B & C didn’t receive request for reimbursement until 6 months into the lease leaving the total amount over $400/person. B & C discovered roommate A hadn’t even paid the gas company and yet was asking for reimbursement. A’s name is on the bill. There is no written record of an agreement to split the bill.