See and I just had the exact thing happen to me, but the other way around. I lease my mare to a girl and she decided to randomly cut six inches off her tail and about 2 inches off her forelock (squaring it off) and did not consult me at all. I know it's just hair and it will grow back, but I happen to love my mare's pretty forelock and thick, long tail and there was absolutely no reason for it. Plus, she flat out just does not have the right to do it without my permission. She is my horse. She has my permission to cut only her mane and only for shows. Period.
There's been more and more things that I keep finding out where they are making decisions and acting as if she is their's and I don't appreciate it as an owner. Some of these things are possibly pretty serious (they were offered meds to get my mare through hoof soreness at a show, and even though they made the right decision and said no, the point is that they don't have the right to make that decision at all and now that their farrier trimmed her wrong they're trying to tell me she may need corrective shoeing). The problem is that we're rarely at the barn at the same time (but I'm always there at least a couple times a week) and there's been very little communication between us, the BO has always been the inbetween and not very reliable at it at all.
But back to the point, the only reason I would ever consult them about anything that I do with my mare is if it may interfere with them riding or showing her, but still it would be more to inform them of it than to ask. With that in mind though, I also realize they are paying to lease her (and I need the financial help for the moment, covering board for two horses in this economy is tough right now) and would not do anything that was not necessary that would hinder them using her as agreed upon. So I can see where it might irritate you to an extent, but if it doesn't actually interfere with you showing him then you really have no right to say she shouldn't trim his anything. You might be able to mention it to her, let her know what your plans are and what you want, but she has the final say so.