Can I witness my grandson’s Lutheran first communion?
Frankly, we cannot encourage attending such an event since it would imply that you believe the Lutherans validly celebrate the Eucharist. I believe that the Lutherans who receive communion good faith will be blessed by God in some way. Even though what they receive is not the body of Christ since they do not have valid priesthood. They rejected the priesthood at the time they broke away from the Church, so their ministers do not have the power to celebrate the Eucharist.
This is why your attendance would be tantamount to bearing false witness that your grandson is actually receiving further Christian initiation by receiving the Eucharist. It would be different if you were attending a ceremony in a non-Catholic Church that is actually valid (Like the Greek or Russian Orthodox churches).
We know this is a difficult thing to do, but this is the cost of bearing witness to Christ. We recommend you gently explain your position without confrontation.