A coworker of mine, an intern like me, is always at least 30 minutes late. But not more than an hour usually. Today the CEO even noticed. But I didn't tell him that part.
Is it advisable to tell him that?
If the CEO/Senior Manager noted their absence in front of you, I would have found a way to tell them.
I agree that you don't know what agreement they have made with their manager regarding work hours. But there were several pieces of information in that incident that need to be conveyed:
This tells them to talk to their manager without you being in a position to judge them or lecture them. It is possible that their manager will know why the CEO stopped by. The manager might also contact the CEO or have the intern contact the CEO, depending on what they decide the next step should be.
Honestly: Mind your own business. Unless you have a vested interest in keeping this person around/helping them out, you're either a rat or nosy.
Consider even that it may be known to the person's manager, and they have a good reason. I have a workmate who is gone for an hour every friday for prayer - and for the longest time I thought he was taking lunches that were "too long". His boss knows and he has never failed to work an extra hour every friday to cover it, but I was gone so I never saw that part.
Leave the managing to your manager and do your job.
If you are close friends outside of work then I would address them as a friend. I would explain that being late all of the time can be viewed as laziness or even theft by employers.
If you are not friends outside work I would leave it be. Unless you have been assigned to monitor the comings and goings of the interns, it is not your responsibility to monitor them. As stated else where they may be an agreement with management about his work hours. Even if not mentioning your noticing it to him is liable to create more problems for you at work rather than helping. So unless he asks you directly if anyone has noticed I would not address it at all.
If this is a problem with your employer then your coworker will be advised that his tardiness is a problem. If not then making it an issue will not benefit either one of you.
As a peer, I would only mention it if there is a direct impact on you. If, for example, your co-worker's tardiness means you have to cover for him, it might be better to raise that with him directly rather than escalate that to your manager.
That the CEO noticed may make your team look bad, but I would say that is a problem for your manager, not you.
Talk to your manager in private about your concerns.
Bear in mind the following factors:
Most likely, you don't - because you're not in a position to advise ("coworker of mine, an intern like me").
Most reasonable course of action for this case would probably be for you to ask advice on that from your manager / senior / mentor. They will take appropriate actions, if needed.