Yup, seriously understimulated dog here. You can tell your roommate it's kind of like giving a kid a single Rubrix cube (the chew toy) and a 20-minute recess once every two weeks and expecting them to be well behaved. If the walks are basic on-leash around the block type walks, that's like if the recess is in an empty parking lot.
I agree that training sessions are a great start to get this dog's mind working. There's a lot of cool ideas for canine enrichment out there too - basically games, puzzles, or novel experiences you can set up for your dog to work on that changes up the boring day-to-day. There's a good facebook group all about it, if you want a veritable treasure trove of ideas.
And I'd significantly increase the exercise. Humans walk super slow compared to dogs, so 20 minutes of walking on a 6-foot (or shorter) lead in the same boring neighborhood is barely anything, even if it was happening daily. If possible, I prefer to go to fields or wood trails and use a long line - waaay more interesting and stimulating, and giving dogs a chance to be dogs out in the 'wilderness' goes a long way towards helping a worked up dog chill out and slow down. Jogging, biking, or taking up a dog sport are also good exercise for a breed mix like this, but if your roommate isn't going to change his lifestyle, he could at least hire a dog walker. If he's not willing to do that... it's obviously not really in your control but it sounds like the current situation is a pretty miserable life for a dog. And for you.