Is a sacrifice obligatory or not for someone who entered Mecca without ihram and then entered into ihram from within Mecca itself due to ignorance of the ruling, knowing that he performed Umrah many times thereafter, adhering to the miqat?

Your friend erred in issuing a fatwa (definition) permitting passage through the Miqat (definition) without entering into a state of Ihram (definition), and claiming that Ihram should commence from Mecca. The correct ruling is that the Sharia (definition) has designated spatial Miqats for Ihram for those intending Hajj or Umrah (definition). Therefore, whoever passes through them must enter Ihram from there. As for those who reside between the Miqats and Mecca, they should enter Ihram from their place of residence. Your friend must repent and seek forgiveness, and all of you must sacrifice a sheep to be distributed among the poor of the Haram (definition). For those unable to do so, their repentance suffices.