My roommate put a hunk of beef roast in the slow-cooker for a good 6 hours. It's a fairly large sized slow-cooker, at least 8 quarts. The meat was cooked through but very chewy, and I never remember my mom having that issue with a slow-cooker.

My roommate tends to put only enough water to get halfway up the side of the roast, and always uses a slow-cooker 'bag' (another thing my mom never did).

What advice can I offer them to get a better outcome?