be sure and tell her that you are not looking to make anyone upset, that you are just looking for a great horse to lease and that you'd be able to look out for her. Say something like "it may not be my place, I just don't think "stupid woman" is treating Reba the way she should take care of a senior horse."
As far as the BO goes, if he/she cares about horses in any sense then this shouldn't be an issue. And if it is and you are already leasing Reba, then there is really nothing they can do about it. If it really gets to be a problem ask Reba's owner if it would be ok to move her to another barn. Suggest that it would be easier to keep weight on her in at less stressful barn.
I hope this helps. Keep us updated.
♥ Gypsy gold does not chink and glitter. It gleams in the sun and neighs in the dark ♥