@avimay I think 5 is a bit young to be leaving the premises for 20 minutes. Does your son have a phone? If there's an emergency, can he call you, or mom, or 911? Is there a nannycam so mom can monitor him while she's gone? None of these are a replacement for a responsible adult being on the premises, but could definitely be a valuable tool in helping make sure your son is safe.
Has mom talked with your son about what he's allowed to do while she's gone? What is okay to do, and what isn't? For example, is he allowed to eat? I wouldn't let my daughter eat if I wasn't home with her (like if I went to the laundry room in the basement of the apartment building).
Also, what solution do you want? Do you want her to take him every morning, or get a sitter? Are you willing to pay for these? My ex wasn't happy when I had a 1 hour gap where my daughter, at 9, would be home alone before I got home from work some days. But I purchased a phone and service for her, and we discussed expectations and she had resources available should something happen. Also, the apartment manager's office is across the road, so she could go there if she needed to, and there were a couple neighbors she could go to for help as well. I had it about as covered as I could. My ex offered to pay for after care, but decided I had the situation handled when he realized how much it was going to cost for 1 hour once or twice a week lol