Hi JAZZ, welcome to the forum.
Are the animals in question visible to anyone else, then it would be easier to stay anonymous as far as the farmer was concerned? Say, near a lane, track, public footpath? You could try the Brigitte Bardot Foundation. You can email them the details, with the photos and dates, and could therefore prepare what you need to say in advance. They will ring you to clarify the situation though, but it's possible someone speaks English, as I see one of the pages on her animal welfare site is in English. They are thorough, and discreet. Especially if you could explain your predicament of not wanting to fall out with your neighbour, but not being able to see animals treated in this way.
It's a very tricky situation, but the BBF could help. They've had to get involved with horse cruelty in this area, so they don't take any animal cruelty lightly and they're happy to step in where Mairies and Gendarmes fear to tread.http://www.bardotbrigitte.com/brigitte-bardot-foundation/