The owner should try and make a police report. There are rules regarding acting on a lien. It does not sound like the BO followed those rules.
Were the horses also on private property? You said they were there for 2 days? If the horses were moved off the BO's property, and the BO went to someone else's property to get them, that is stealing. It doesn't matter if there is a lien or not. It is a matter for small claims court, it doesn't give the BO the right to go get them.
" Under the laws of most states, boarding stables do not automatically own boarded horses merely because the owner has fallen behind on payments. The answer would be different, however, if the stable followed - to the letter - all of the requirements of the applicable state law. Laws that apply in these situations are known as "agister's lien" laws or "stablemen's lien" laws.
For example, under Michigan's law, Michigan Compiled Laws Section 570.185 (and the sections that follow), a stable must wait until 9 months pass, without payment, before holding a special public sale that a sheriff's deputy or authorized court officer must conduct with the purpose of selling the horse to the highest bidder. The top bidder pays the money and wins the horse. One month before the sale, a specially-worded notice must be sent to the owner using language that has been supplied by the statute."