Washing with beer can help with itchy skin and skin problems. My mom washes her hair in beer occasionally to help with dandruff and itchy scalp. I could see if the boarder brought the case of beer to rinse the horse with, but not give the horse 1 a day. I would think a supplement and/or a misting fan (easily DIY’d) would help more than a brewski.
As a side note, occasionally the guys who come in my barn share dribbles with my horses, who typically sniff it and walk away. I don’t like that they do it, but am happy knowing my horses turn away from it.