If your friend lives in a Tier 4 area then they should remain within the Tier 4 area and not travel. If not living in a Tier 4 area but need to travel through LHR which is in Tier 4 then they "should consider the public health advice in the country you are visiting".so travelling in this situation is permitted. I don't know who would do checks at Heathrow to verify, maybe at airline check-in or passport control.