One pipe steam radiator in old two pipe system

Good morning all. I am a noob to this site. Here is my question, we have a two pipe steam system in an 1939 three story building. The building just got renovated and the old oil fired Boiler replaced with a Weil-Mclein series 80. during the renovation a partial 4th floor open room was added and an undersized radiator was added up there to just prevent freezing up there.

Our plumber piped it with one line from a third floor radiators steam riser and the condensate will return down that line. Is this a problem? With the little I know I am concerned we will have problems with water hammer and condensate in the steam side of the system. I would like some advice on what to do as we are about to seal up the floor on the new fourth floor and lose access to the piping if we need to change this.

let me know what other information the group needs to give me sound advice. There are other issues with the system but this is the most pressing as we have flooring guys showing up tomorrow...



Thanks in advance.