Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Have you asked them if this is the only shirt with the problem or is with all of them? It doesn't look consistent all over which is what I would think if it were an undercure. Just my opinion though.
That looks like some sort of abrasion mark, with some carbon as well. This is very similar to what a shirt might look like if it gets hung up in your dryer somehow for a bit too long.
It could have also been hit with an aggressive solvent... similar to - Gasoline...We have seen shirts where someone was using industrial solvent type cleaners and got it on their shirt - then blamed us for a bad print