Question. Since your barn owners are aware of Relevante's extreme reaction to this filly, why are they continuing to put her next to him? I realize you said they ended up moving her to a round pen, but she is also reacting to him and continuing to try them together , with a fence between is asking for one or both to get hurt.
Stallions and geldings often will bluff when meeting other horses, but mares are usually deadly serious. I pasture bred mares with my stallions. always after observing their behavior in which they were initially separated by driveways, barn aisles, , fences , etc.and there were times I had to abandon the idea of a particular mare and stallion combo. I chose not to hand breed my stallions, but am good friends with others who do and I can tell you for certain that there were never any injuries to the mares during breeding. If there were any injuries to either horse or human, it was to the stallion or stallion handler as they approached said mare. It's why breeding hobbles, twitches and sometime sedatives are used for mares. I had the most sweet , lovable mare that little kids and husbands could ride, was a beautiful, easy going momma with her babies and with humans who were helping at foaling, who absolutely beat the crap out of Tipperary for not approaching her in the pasture as she preferred, despite the fact she was in standing heat. I mean double barrel and she waited until he was directly behind her to mount before she let him have it with both feet. Mares will also double barrel the stallion as he slides off after breeding, my other stallion was almost knocked completely over backwards by the most gentle , sweet mare. Both mares were experienced breeders, the first had two previous foals out of pasture breeding and the second had six foals, from a mix of pasture and hand breeding.
My point is that this may be a bad combination of personalities, no matter what. This mare should be out of heat and should be more tolerant of other horse's attention at this point. The fact that she is not tells me that they should not be in direct contact with each other. Unless Relevant has done something to break a tooth, etc. he should be eating his hay. The fact that he is fascinated with this filly may eventually wear off if she is not accessible to him, but he may be spending as much time as possible where he can see her. This is somewhat unusual behavior for a gelding, not unusual for a stallion ( I have friend's whose stallion paces his fence line for hours in a day , even when there is no change in mare placement on their farm, etc, he almost never needs hoof trimming and is horrific to keep weight on) . I would try to ride this out as long as the farm owners do not keep the filly next to him. I would put his hay in his turnout where he can see her, but the hay is in front of him. I would guess he will eventually decide to give up on her if he has no access to her and will start eating again.
I would, as a precaution, have plan B in my head. Plan B would involved moving him somewhere else if filly is a permanent boarder and his behavior doesn't change.
Dawn Wagstaff and Tipperary
Saline, MI 2003