I've not read all the replies so apologise if this has already been mentioned. Sounds like the horse in question is either a rig (has retained some testicular tissue) or is a false rig (was gelded correctly and completely but stallion behaviour is in his mind).
The owner can sort this out easily enough by getting the horse blood tested to find out.
This happened with a previous horse of mine, who I was told was mounting other geldings - we had about 40 geldings out in around 60 acres or so. My gelding had been gelded at 7 so fairly late. I spent hours down the field with a camcorder trying to tape any activity to show it wasn't my horse but of course none got injured whilst I was there. He had a blood test although I was certain it wasn't him causing the injuries to the other horses and he was declared ' a false rig'. The vet said I could use a product unlicensed for use on horses if I signed a disclaimer. It was called Depo Provera. So he was injected with this so he could go back and join the other geldings as he was on permanently stuck in his stable 24 hours a day as the YO wouldn't let him out with anything.
My horse and I were at a show one day and one of the liveries horses got absolutely battered in the field. Turned out it was another horse mounting the horses and injuring them. I left the yard as I felt I'd lost all faith and trust in my so called friends who had made my life so miserable saying it was my horse all the time when it obviously wasn't.
My horse dropped dead about a year after the final 3 injections with a suspected heart attack in the paddock. I will never know to this day if I signed his death warrant by agreeing to get him injected with this hormone. I spoke to my own vet years later and he said that horses gelded late often have issues with their heart and can be at increased risk of sudden death.
Oh what a sad story so sorry. He has lived with other mares in the past with no problem at. I am hoping it's because he's new and we have only had him 7 week. But if doesn't calm down I will get him tested. Xx
