It depends on the type of pentecostal church. I'm currently searching for a new church but for the last 15 years I've attend an Assemblies of God church which is under the pentecostal umbrella and there is absolutely no dress code. Pants, jeans, t shirts, dresses, red, black, purple, etc. People even wear shorts and flip flops. Most of the women wear makeup and cut their hair. Even though its a conservative church, (mostly white republican) it's fairly laid back, the boys and girls from the youth group even sit together.
The person that invited you should have given you a heads up on the dresscode if there were any really big no nos at his or her church so its mostly likely a come as you are at least w visitors. But your friend is the best person to ask. You should also ask your friend about what to expect during the service. I wouldn't want you to be unprepared. Have fun though, I hope you enjoy it!
