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?
It doesn't matter if it is covered or not. The inside of the slow cooker will be warm enough to cook the meat. Braising (not submerged) and simmering (submerged) are two methods which both can lead to good results.
The "very chewy" result sounds like choosing the wrong type of meat for slow cooking. If it was a real roast, then this is the obvious problem. Roasts should be roasted, not slow cooked. If it was an actual slow cooking cut sold under a bad label (these things happen), then probably it was cheap meat from an animal grown up without movement and slaughtered too young. You have to choose tough meat with lots of fascia streaking for slow cooking.
If the roast was dry, chewy, seeming over cooked. Likely the exact opposite occurred. It probably needs to cook much longer. It will fall apart when it is fully cooked. I finally learned this last night. My roasts always came out dry and even hard to cut with a steak knife. FINELY figured out the problem!!! Our 4 pound chuck roast ended up cooking for almost 24 hours!! Probably was done sooner but it sure came out great!