If a Lutheran couple asked a Catholic friend to read a Scripture passage at their Lutheran wedding service, could the Catholic friend do so?
Regarding Catholics participating in readings at non-Catholic churches, the Vatican document Directory for the Application of Principles and Norms on Ecumenism says this:
In liturgical celebrations taking place in other churches and ecclesial communities, Catholics are encouraged to take part in the psalms, responses, hymns, and common actions of the church in which they are guests. If invited by their hosts, they may read a lesson or preach. (PNE 118)