Stay confident, Cynthia. Tell the d-i-l's sister and roommate to stay isolated as much as possible. When I had a scare and isolated myself from my husband, he put a small table outside the door and placed meals, etc. there for me to retrieve after he had left. Of course, cell phones provide good, clear, well-distanced communication. ...even from room to room. The roommate should have a spray bottle with a bleach solution to clean anything she touches. ...especially if they'll still have to share a bathroom.
Some new studies out of England are indicating that aspirin, yes, ordinary aspirin can be helpful. They might both start taking it prophyllactically. And melatonin, as well, shows indications that it MAY provide some benefits. It could be that better sleep helps the body to cope most effectively but they also speculate that melatonin is something taken by more health conscious people who are less likely to let themselves get exposed and may have better recuperative powers. That much isn't clear to them yet, merely that there's a statistical dip in instances in people who take it. In any case, melatonin isn't something that could do any harm.
. Before roommate had symptoms. We know how that can go. The waiting game starts now. 
I don't have high hopes.
I’d have to disagree with the aspirin advice though. They should check with their doctor before taking aspirin as it can cause harm in some people. Just because it’s over the counter doesn’t mean it’s harmless.
