Only if that is within a company's policies - would not recommend automatically giving the Resource's Manager the ability to submit a timesheet. Maybe, only if 'submit' right was separated from 'edit' right.
There are bad managers out there - had one that signed his name to my work (he was fired) and just heard same story from one of my son's friends (friend is leaving company, for this and other ethics related issues). Such managers may not like what his resources are booking to and make changes.
One would like to think that such managers will get called out in good companies, and in bad companies, it doesn't matter.
Most managers are honest. An employee leaving is an infrequent event, where a business policy can be set up easily enough and CA PPM configured to fit it (e.g. admin assistant may submit, or if manager submits, the manager's manager must approve).
Just don't want automation to make it easier to cheat for the few bad managers that are out there.