My current OTTB won't gain weight for the life of me, he is fed twice a day and he looks malnourished sickly BUT HE'S NOT and he is pastured with 2 of my other two horses who don't look underfed. I'm fairly sure if he was pastured alone I would be getting in trouble.
Just play it by ear, keep an eye on it and if the situation gets worse, call someone. There isn't much else you can do if you don't personally know them.
16 year old TWH Mare named Ginger
3 year old APHA Paint Gelding named Fox.
RIP Evie (Clydesdale) 7.29.09 & Magic (OTTB) 2.23.10