I think it is under the Treaty of Vienna, which defines refugees and how they are to be treated. There are provisions for dealing with them, they SHOULD claim refugee status in the first 'safe' country they arrive in. However, the remainder of the EU are getting 'fed up' dealing with them, the majority of the refugees want to come to the UK and so the EU countries are giving them free passage to the Channel coast.
It's not like we can let them catch Covid here and then send them elsewhere so warning Syria of the dangers wont help much.
Perhaps we can persuade all of the refugees around the world to return to their land with our pestilence and infect all of the soldiers to stop the wars once and for all.
You two could never be diplomats. lol They should be stopped on the route. Turkey maybe. Or pack them all off to Australia. lol