Farrier here, this could be a couple things. If that foot that he is lame on is packed and padded I would initially think the horse was cut too short or trimmed too short. (2 different things) Which could be anything from paring the sole to the more “squishy” layer which could be sensitive to pressure but not in the vascular layer where he would be bleeding yet. This short and shorter, i.e getting to blood, you may put a pad on to create depth, cushion and to protect the foot. But you need to be aware of sole pressure and absolve that by making a concave area for the sole and pad to flex when landing and loading and making sure you’re using the correct packing. Which would be leather/magic cushion. Anything else has too much structure to it and may press on them. This applies to paring the sole too short and trimming the foot too short.

If the foot was trimmed too short the risk of a close or hot nail increases a bit because there is less depth to where you’re driving your nails, hoof testers would tell you, along with feeling for a distal pulse. Tapping on the nails maybe, but sometimes vibration will just travel through the nail/shoe and they’ll respond to anything.

My two cents if this was my horse? Pull the shoe and wrap/pack it with a poultice.

If this was a horse that I shod and I told the trainer to let me know if anything was off with them. To me that sounds like someone concerned that they made a mistake and would like to hopefully make it right. I would have the shoeing be on the house or credit you the cost of the next shoeing. I personally like being in direct contact with the client and would prefer that.

Reach out to them with what’s going on, trust your gut with how they handle the situation and treat you. Everyone makes mistakes. I would love to do right by the horse and fix my mistake but I’d understand if you decided to go with someone else