It depends on whether you are talking about one or more people. When talking about one person, you would use 'who has' but when talking about more then one, you would use 'who have'.
I know Peter who has been very kind to me.
I know Peter and Jane who have been very kind to me.
Both are correct. Who has comes with singular person and who have comes with plural person. For example : Who has taken the chair ? Who have not done their home work ?
Could be either.
Who have you helped across the road today?
Who has been helped across the road today?
depends on the context.