PO leaving company
1 - What we can do if our PO has decided leave company in the middle of project?
2 - What is the advice / rule in Scrum?
Did the PO work for the client or the supplier? Are these the same or separate organizations?
I don't think Scrum advice anything on it, this is a typical resource management issue. There are multiple options - your best bet would be to have a team member (BA) do the PO job till the organization find out a new PO for your team. Someone who owns the product or whose money is getting invested into the product should be the PO.
> I don't think Scrum advice anything on it
There's no specific advice, but by definition the Product Owner owns the product. This means that if the PO leaves an organization the product can, in theory, be expected to make the journey too. Of course this depends upon whether or not the PO is an entrepreneur, and not just a hired representative of stakeholder interests.