Within CONTEXT, it is fine.: Two men came to see me today. The one who came earlier ....etc It is perfectly fine to use 'one' in this sense. You would not use 'man', as that would be tautology.

If possible, you should avoid using 'one' to refer to another person. It can sound quite disrespectful, as if you are objectifying the person. It is preferable to say 'the person', 'the man' or 'the woman' ( or more politely, 'the lady'). In some circumstances, you could refer to their role in that context e.g. 'the customer', 'the passenger'.

You could say, "the person who came earlier showed me this". But if it's someone both you and who you're speaking to know, then you should use their name. For example, "John showed me this earlier".