What happens when an employee who has become a co-owner leaves the company?

Normally, employees are obligated to offer their shares to the organization (the company) when they leave the company. The organization then still has the choice whether or not to repurchase these shares. If the organization chooses not to buy back the shares, the employee is usually given the option to sell the shares to other employees. If other employees also do not want to acquire the shares, the (former) employee is left with them and typically has to offer them again a year later.

Whether a departing employee actually has shares that they are allowed to sell, and whether they can do so at the current price or must do so at the original price, all depends on whether there are any restrictions on the shares and whether the employee is classified as a "good-leaver" or "bad-leaver". If there's still a "vesting" period on the shares (meaning not all shares are fully owned by the employee because they haven't completed the required time at the company), then the employee can only offer and sell the "vested" shares. If the employee is classified as a "bad-leaver", it may be the case that they only get the original price for the shares or even get nothing at all and must return the shares for free.