Your friend says that inertia is a force that keeps things in their place, either at rest or moving at a constant speed. do you agree?

No

Explanation:

The error in the sentence is that inertia is not a force.

In fact, inertia is just the tendency of an object to keep its state of rest/motion if the net force acting on it is zero. As stated in Newton's First Law, also called Law of Inertia:

"An object at rest will remain at rest and an object in motion will continue its motion with constant velocity if the net force acting on it is zero"

This law basically tells us that as long as there are no forces acting on an object, the object mantains its state of rest/motion. On the contrary, in order to change the state of motion of an object, we need a net force different from zero.