Is this a good sentence?Do I need to insert an "a"
He's still good friend to me
The other construction, with 'to', is as in sentences such as:
She has been a good friend to me over the years.
He's more than a friend to me.
'friend of mine' indicates that you include him in the circle of those people you regard as 'friends' - or, of course, 'a friend of hers' etc.
''friend to' means that the person is behaving/has behaved towards someone as a friend might do/is characterized as doing.