Depends on if you're talking about moral responsibility or organizational responsibility. I'm a big fan of employee accountability.

Well as an employer, legally do we have to take any action if we suspect or can prove an employees has a problem

you may want to do something to prevent the employee from being a liability. Time to talk to lawyer maybe.

If its having an impact on performance, you can call them out those issues. It's best to make employees aware of options and help available.

we couldn't see any difference in work.