Barn Owner's horse becomes aggressive towards my horses when around humans

Recently the barn owner and I were chatting while petting his horse. My horse quietly walked up behind me and before I even realized he was there, the mare lunged at him making contact with my hand. Luckily she only got it wet with saliva and did not bite me. The barn owner backed her up but did not do much else.
Then just last week, my 11 year old daughter, who is quite tall for her age, was leading one of our horses out of the pasture when once again the same mare came up from behind with ears pinned back and tried to bite him. I got after her but doubt it made much of an impression.
They have this horse on a lease and are planning on buying her. She has had a lot of training. I need suggestions on what I can say or do.

Some horses are just jerks. They know its a good time to bite.

Yes, well the barn owner has small children and one day someone is going to get hurt.

What I would do is have a friendly chat with the Barn Owner about this horse and together come up with some suggestions on how to deal with the issue. I'm sure they don't want anyone to get hurt, or have to deal with a lawsuit or nasty publicity because of this mare.

If the Barn Owner wants to do nothing, then be aware this is an aggressive mare and two people should go out to get your horse; one to lead and one to chase off the aggressive mare.

I would not be having casual conversations while in the field, since you don't know when this horse is going to come over.

I agree, talk to the owner.

Where I keep my horse now we have a mini that tries that stuff with my horse while he is tied or while I am leading him. I have permission from the owner to discipline him, so I make it very unpleasant for that pony to come near me while mine is attached to a lead. For a while I had my hubby come out and hold my horse while I got after the other one. He tends to leave us alone now, but for awhile I had to chase him away with a whip. Either that or I caught both of them, and then had hubby hold the mini while I got my horse out of the pasture.

Tell him not to put your horse in the same pasture, Why would you even tolerate this ? Personally I would never board somewhere that horses from different owners get put in same area. Just to much stuff like this happens.

Really? I don't think I've ever seen a horse stable that has enough room to keep everyone's horses separate. Every stable I've seen throws them all together...

It doesnt take extra room the pasture just gets divided in smaller areas. I board 2 weeks out of the month and I picked this place specifically because each owners horse has it's own area. Maybe boarders should start asking for this and voting with their feet.
I constantly see posts, or hear people talking about their injured, or disfigured horse, or agressive horse problems from horses owned by someone else. I know things happen but Id rather it be between my horses and not caused by a mystery horse and get a shoulder shrug when I show up and my horse is injured. No how no way would I deal with that.

Joe, there is a big difference between boarding two weeks out of the year and boarding all the time. My horse lives with 4 other horses without issue. They will occasionally have an argument, but nothing that causes injury or disfigurment.

I care about my horse as much as you do yours, but IMO, they are not babies and do not need to be handled with kid gloves. Horses would prefer to be with the herd.

Obviously there are some horses that are extremely aggressive and in that kind of situation, I would want that horse isolated....not mine.

So, is Joe boarding his horse 2 weeks out of every month or 2 weeks out of the year? It's a big difference.