while you have your horse at a trainers and a cough develops 4 weeks into the training and he doesn't bother to tell you until you pick her up 10 days later and she's still coughing, you spend $600 trying to find out why and come to find out later after a BAL it's a foreign object should the trainer be held responsible for the bill? 

Depends on how good the trainer is and if you want to burn that bridge in the future. 

I doubt seriously I'll be taking another horse to him. Just trying to find out if he should be liable. I'm afraid he gave her some of his round bale hay that has been left outside but no way to prove it 

I would say he probably didn't do it on purpose and probably didn't think it was anything other than a cough. I would let it go. 

i know he didn't do it on purpose but he didn't bother to call me to let me know she had a lingering cough and it's been going on for a month and $600 in vet bills trying to find out why

While I would be irritated he didn't say anything, I would chock this up to the normal equine death wish. I highly doubt there was actual negligence involved, just a freak accident that horses are always prone to. Hope your horse is ok.

While I don't think he is liable, I would be pretty unhappy that he did not keep you informed.  I would find a trainer with better client communications skills going forward.

I feel he was liable. Poster paid to have her horse trained & cared for. Trainer had the responsibility to take care of the horse which includes feeding along with shoeing, etc. Owner pays for shoeings as well. If he put out round bales, he put out crappy ones & crappy feed should never be fed to someone's horse who they are paying to care for. We have never found anything nasty in our hay. We pay for good quality hay by reputable growers. Not cheap but we want quality not crap & trainers should be the same way.

As the owner of a barn that has horses come and go, usually young ones for colt starting, I generally shrug off a cough.  It's like of kid starting kindergarten, IMO.  Generally they are over their cold before an owner comes to pick them up.  We are in a wild fire and wheat harvest area.  Every late summer, everonyone is always coughing.  Moving from a wet climate, it use to make me paranoid.

NOW, I do pay attention to what causes the cough (Exercise, feeding etc.), how long they cough, what kind of cough (dry/wet).  We had one start to choke shortly after drop off, owner said give banamine and keep in touch, I was panicing.  I pay attention to coughs, but generally are just colds for unexposed horses who haven't built an immunity. 

If I have a horse with a persistent cough (no temperature)  I'll let the owner know and give him the option of putting the horse on some Uniprim or cough syrup to see if it will go away, I can take him to the vet if it doesn't go away easily. Young horses are really prone to upper respiratory infections when they go into a new barn. 
if the owner would like to have the horse looked at by the vet , or the horse is also running a temp or I feel like it's something that needs to be looked at by the vet, I'm happy to haul thr horse to the vet.

***no matter what, the owner gets notified, owner pays for meds and or vet bill. Owner gets to be part of the decision process. 

some trainers totally ignore sickness, some don't take regular temps, some just ride the horse , however he is that day. Some are clueless as to medical problems.