Can I Attend the Baptism Service of My Lutheran Friend?

Yes:

According to the local situation and as occasion may arise, Catholics may, in common celebration with other Christians, commemorate the baptism which unites them, by renewing the engagement to undertake a full Christian life, which they have assumed in the promises of their baptism, and by pledging to cooperate with the grace of the Holy Spirit in striving to heal the divisions which exist among Christians. (Directory for the Application of Principles and Norm on Ecumenism)