Ask HN: Are my parents liable for an unsecured network?

My parents wifi network became unsecure after their provider was acquired by another network. A tech support agent rang them and guided them through the reconfig process to get on the new network but didn't tell them that they needed to set a new wifi password and that their network was unprotected. As a result someone racked up charges of €80 ( > $100) in excessive data usage fees. I complained to the network in question and got the "not our fault" line. Both of my parents are over 55 and wouldnt have a clue about wifi security. Am I being unreasonably thinking that the fault here lies with the tech support agent who guided them through the changeover but didn't re-secure the network?