They're booked into the Kerr Historico hotel. Never heard of it until today.
It looks like it was just launched: https://www.kerhistoric.com.ar/
I saw on the map where it is: on the border with Puerto Madero. I'd rather go there at night than in downtown San Telmo.
It is true that not all areas of San Telmo are sketchy after dawn, but you need to develop a radar to smell the sketchy Buenos Aires' calles. An empty street at night in Europe doesn't mean anything, here should steer you away immediately.