Frankly I would move the horse if she asks again or you have concerns she might use the horse behind your back. I might even go so far as to ask her to sign something saying she will not use your horse for lessons or allow anyone else to ride him for ANY reason without your express written permission. Keep a copy.
I am far from opposed to letting my horse be used for lessons. However, it has to be the right horse and the right situation. My 20+yo mare's sole purpose in life right now is playing beginner lesson horse at my friend's barn. She enjoys it, its an easy job for her in 'retirement' (mare can't actually retire and do nothing...she played pro high goal polo all her life and goes bananas and is just evil to the other mares in her field without anything else to do) and she's good at it. She's now the barn's favorite lesson horse. She's also with a friend I trust to use her appropriately and I rarely see her other than to come out and give her cookies a couple times a month. I also know she's a stubborn, opinionated witchy old mare who will not let her self be overworked or abused. She will protest and start tossing people (which is largely why I own her now...). HOWEVER, she's my second horse, not my primary mount and is old enough that no amount of bad riding is going to screw anything up. She is what she is at this point.
My gelding? Another story. He is MY horse. Do I let people on him sometimes? Sure. Would I let a friend take the occasional lesson on him or toss friends/family on for 'pony rides'? Yes. However, I bought him, I trained him, I made him the awesome horse he is so *I* can enjoy him! NOT so he can become everyone's go to lesson horse.
I would not keep my horse at a barn where I didn't trust my instructions not to be carried out. I would not keep my horse with a trainer who tries to pressure me to do something like that that I don't want to do. I wouldn't necessarily be offended by being asked once, but if I said no, I expect that to be the end of it.
greenapples, I am sorry your trainer is persisting after you said 'no'.
The question I would ask is why doesn't this beginner rider buy or lease her own horse? She will progress more quickly with her own horse.
If the trainer is still after you on this subject my feeling is that she is not looking out for your horse. An older horse needs that tlc care to keep him/her going as you said, and his health and well being should be trainer's primary concern, as well as your satisfaction with the arrangement.
I have had others ask to ride my mare, the short answer is 'no' (only a designated person for when I am away on travel, and she and I have worked out a good routine for her to ride the mare). It doesn't matter that they are better riders than I am, it is that she's mine and we have a relationship.
'Stuff' happens with horses and if she was riding him when he took the odd step how would you feel?
You should put this back on the trainer and tell her you're unhappy with the discussion, and why is she doing this to you?
Thank you Alpha Mare. Your post summed up pretty well what I have been thinking.
The rider has only taken a handful of lessons from this trainer, so I'm not even sure what her goals are. I've met the person twice, and I don't know her well at all, and certainly not well enough to feel comfortable loaning out my horse for lessons.
I think what irritates me the most about this whole situation, is the trainer's attitude about it. No discussion about using Dobbin for lessons, just a casual asking if she can "borrow him for lessons" sort of thing. No offer of compensation for vet care or even a nominal fee for using him or even a discount off of my own training.