[peterwwillis]

The dumbest, cheapest, simplest option is e-mail. No matter who they are, if they're an actual company, they have to have an e-mail provider for their domain. This means (at the very least) they can create a single account that will accept e-mailed issues, and forward to all relevant users. Track the issue in e-mail thread replies, which always has the latest updates.

There may be provider-specific productivity features, such as G Suite's Collaborative Inbox, or Exchange's Public Folders. The IMAP Protocol also supports shared mailboxes, so any IMAP server that supports this will allow you to collaborate on issues sorted into folders that multiple users can access. Some open source mail servers definitely support it. Once they pick up a real issue tracking system it will probably support issues created via e-mail, so it can be seamlessly integrated later.