I can never truely understand owners like this,probably why it annoys me so much.Gotta feel sorry for the poor pony stuck in the middle of this(not that means you should continue to look after pony free of charge)
Its a bargain. She should snap your hand off!! The email sounds exactly right (the re written one, again, bluntness is me!!) so hopefully that will sort it for you. It is not acceptable for them to just pay the livery and leave pony to it. Or rather, leave you to it.
I don't think you are charging over the odds with that, your friend is wrong. The 'owner' is neglecting the horse, I assume full grass livery is something you don't normally do, I have never heard of it before (other than in such as retirement livery) so I wouldn't imagine there is a price in the area for it. If she can't be bothered for whatever reason to go twice a day to check her pony is ok then she should pay someone else to do it. I assume as a yard owner you have other things to do, perhaps your own horses to look after, so why should you have to make extra time to take on her responsibility without her paying a fair price for it. I hope she takes you up on it because I have no doubt you have been doing it anyway without charge for the 9 months the pony has been with you and at least the poor horse will benefit.