It doesn't fall under the allowable reasons for leaving home.
Getting food? No Medicine? No Medical appointment? No Caring for someone? No Essential work travel? No Exercise? No Fleeing harm/potential harm? Possibly but that's stretching it a bit.
He should "out" his flatmate but of course he does have to live with the person.
There's also the question of the actual travel. If he drives then there could be an accident taking up much needed resources, and of course bringing people into close contact.
If he takes the train it's even worse. The number of people he could/would come into contact on the journey.
He should stay where he is, and isolate from his flatmate as much as possible.