My friend wants to come to check out our church as she is thinking of switching synods, but she wants to know what is appropriate attire for a service.
As you are a member of that congregation, you are really in a better position than I to explain the culture of your congregation. You can explain to your friend how you normally dress for a worship service.
You can explain to your friend that our churches do not have dress codes. The area and culture of some congregations lead to more formal attire on the part of worshipers, while more casual attire is found elsewhere. The age differences of worshipers in the same congregation also leads to a variety of attire.
Do make it clear to your friend that the attitude of the heart (1 Samuel 16:7) is much more important than how we dress for church.
Perhaps you can sit with your friend and guide her through the worship service. That would make for a comfortable first-time worship experience in a new church.