If a user using HighSide loses their keys for whatever reason (Whether their computer is reset or something becomes corrupted on their machine) and they don't have a backup of their keys, there is no way to recover the keys. HighSide cannot get that users information back and the user will require an entirely new invite to HighSide. You will need to first deactivate their old account, then you will need to send them an invite as you did when they were first joining the team. The user will no longer have access to any old messages they needed but they will have the same access to documents they had. The user will also need to join any channels both public and private they were previously in (They will need to be added to private channels again).
Deactivating the users account:
In order to deactivate the users account, as an admin, you must go to contacts and click on the user you wish to deactivate. From there you can Scroll down to the bottom and you will see 2 options, if a user has completely lost their keys the best option is to deactivate and remote wipe. This means any device where that user was logged in on will be wiped of that users information and thus access to the team.
Reinviting the user:
As the admin of the HighSide team, go to contacts and in the top right you will see a blue button with text that says "Invite People" clicking this will allow you to input a name and email address to send the invite to. The user will then have to accept the invite as they would have when they initially signed up for HighSide.