The priest IS wrong when he claims he doens’t have to follow the GIRM.
**As for the Bishop, it depends on which part of the Eucharistic Prayer is in question. The Bishop determines standing or kneelling for the part after the Agnus Dei and all the parishes in the diocese should follow whatever he has decided. However, the Bishop doesn’t determine the posture for the Consecration. ** It’s always kneeling absent constraints such as physical inability to kneel or not enough room. If the Bishop has directed everyone to stand for that part of the Mass, that would be exceeding his authority.
While it might be a useful exercise in humility to go along, one is not obligated to cooperate with wrongful direction.
The norm for the universal Church (Latin rite) is kneeling for the consecration. The US norm is the definition of the time to kneel from after the Sanctus until after the great Amen. If another country has a norm for standing during the consecration, the conference of bishops would have requested and been granted a variance from the GIRM. There is no such approved variance for Latin rite Masses in the US.