
There are no rules to prevent this. Whether it is ethical hmmm I've competed under friends in sheepdog trials(different to breed showing of course) sometimes. I won sometimes I didn't.
Could depend on whether the handler is a professional, a friend or just someone who handles other dogs for the pleasure of doing so.
Sometimes at sheepdog trials there will be an informal show were the "judge" assesses the dogs on whether they like a dog. No breed standard just personal preference. People do occasionally run another's dog for various reasons(like the owner has a broken leg which happened to me)& they may then run a dog at another trial under that owner.
If the rules allow it then it will happen.
What you are inferring is that some judges are dishonest & pay back favours with places at other shows. Some judges will always be dishonest TBH but it is usually more subtle. A friend who shows their dogs can look at a schedule & correctly forecast who will win. They even did this years ago with BIS at Crufts & they were 100% spot on, others thought my friend was wrong when the dog stood 2nd in it's class, however the judge was just about to confirm the places when she changed the 1st & 2nd places allowing the subsequent winner to go on to BIS. The breed, group & BIS judges were all in a small circle of exhibitors & judges BTW.
You can enter or not if you suspect a judge isn't honest it's up to you.