I was thinking rust until I saw the picture. The concrete at my shop was poured in march and if I spill even a drop of sh it rusts it. The downspouts of the gutters also have fairly large rust spots under them. I’ll take a few pictures next time I’m in just to prove it. You may have him try oxalic acid just to see.
Update: just got done watching concrete dry 
I even post treated a random square in the lot just to see if it would cause any discoloring in the future. I don’t know why this concrete rusts so easy just from the water from the downspouts. I’ve even looked on the roof to see if there was a source but nothing. You can tell the inside of the gutter itself remains stain free but the concrete is obviously staining. A decent oxalic mix and a light brushing with a deck brush to keep color consistent took it right off. I even posted a couple pictures of some random stains in the parking lot that would become far more noticeable after cleaning. I would still try treating it as rust just to eliminate it if nothing else. But it’s my birthday and I was doing this wearing my good clothes. I wish him luck with it rather way!
