It’s not professional to address clients as “sweetie”, “boy”, “private”, “old goat” etc. Along the same lines, it’s not professional to talk to someone who does not work for you as if they did.
The boss-employee relationship can have good perks on both sides but is also based on many more mutual obligations than the client-expert one. For instance, workers get paid to do what they are told and if what they do turns out to be futile or wrong, not many people will blame them for following orders.
A specialist-for-hire is almost the opposite: they are the ones that tell the client what needs to be done and are on the hook if it turns out to be wrong. They get no insurance, no days off, and no guarantee of future work, plus all of the blame.
Neither “deal” is superior, but they are as different as night and day, or, more to the point, different like men and women. Neither is an offensive term by itself, but calling someone by the wrong one rarely ends well.
That means that there’s a big list of things that can be said to an employee but are a huge red flag on fire if said to you. First and foremost “could you mentor X to do your job“, followed by “you are promoted” and anything beginning with “while you are at it, could you just…“
Calling out inappropriate language like that as soon as you encounter it is the only way to keep things polite and functional in the long term.