What are the boundaries that indicate of balance in personal and professional relationships?

It depends on the size and nature of a business, and the history of the relationship. If there is a personal relationship it's best kept open so everyone knows about it. Like I once worked in a medium size family business (about 300 employees) where 20%+ of the staff was family. It was fine because everyone knew of the relationships and respected the bond between family members. Same for professional-client type relationships. Like Doctor-patient relationship. Like if your doctor is also your neighbor and you get friendlier service it's understandable if the relationship is public. If your doctor is also your secret lover and not your neighbor that may not work out too well.