My son has a roommate now that shares meals, so they could buy some Costco basics, like eggs (2 dozen are the price of one dozen in the grocery store and won't go to waste with two people eating them), cheese, and non-perishables like canned tuna. Of course, the challenge is always to avoid the impulse buys. Even $10 or $20 savings a week could add up to some money over time.
Yes, I realize I spent much of my marriage shielding my kids from their father's unreasonableness - but they still felt it. I feel bad that their relationships with their father are not better, but it is what it is.