Firstly from what you have said I am going to assume your friend is speaking in good faith about the miscommunication on the confidentiality of the information as opposed to trying to retroactively cover themselves after intentionally betraying you, the later case (in my opinion) cannot be solved interpersonally and the solution would simply be to stop talking to this "friend".
I cannot comment on what is normal in the whole world, but I at least do not assume any information is in confidence unless either I am explicitly told so, or the content of the conversation make it very obvious (for example a friend talking about an ongoing divorce or my spouse talking about an ongoing legal case).
The first case is extremely cut and dry, no reasonable person will misunderstand if you say "This is private so please do not speak to anyone about X".
The second case is not clear cut at all, what each person considered implicitly confidential varies, and it is usually best not to rely on what a particular person will consider private unless it is someone you know very well and have discussed ground rules about this before.
If a friend spoke to me about for example, a concert they had gone to and enjoyed, I would assume absent of instructions otherwise that it is fine for me to talk to others about it, but on the other hand, if a friend spoke to me about a debt problem they had I would probably assume its private, but I have had clashes before on where to draw the line between these. I have a relative who (like your friend) assumes almost anything is fine to share, and another relative who takes your position that nothing should be shared by default. Neither of them are "wrong" as such, they merely have different norms and assumptions, and this must be understood to communicate effectively.
What to do
In most cases, I would suggest simply learning to explicitly ask if you wish to talk about something highly confidential, firstly this makes it very unambiguous it is not OK to share what you are about to talk about, secondly it means your friend can refuse to continue the conversation if they do not want the burden of keeping the secret (It isn't fair to assume someone is fine keeping secrets without asking, this goes against some peoples morals, particularly if they have very strong beliefs about telling the truth or trusting a friend or partner with all matters).
If the friend is very very close and you speak about confidential matters frequently, in a few cases I would have an explicit conversation about what is important for you to keep secret, and what you don't mind being shared. However this still relies on them understanding and interpreting your boundaries, as well as remembering them, so I would only try this with someone you are extremely close with.
There is no norm about this, always ask before bringing up a private matter explicitly, and only continue if they understand and agree with your terms of confidence.