First you have to clearly communicate to everyone involved that the platform is not their place to showcase their awesome musical talents. It's the place to enter into the throne room of the Almighty.
You all are but humble servants blessed to be given the opportunity to worship the King, all thanks to Jesus.
As such, an attitude of humility is PARAMOUNT for everyone on stage, from "leader" to whomever is serving with yet under the authority of the leader.
That said, if the leader says "I need you to play x, y and z to the glory of God" then they best do that and do it with joy. Anything less is with a wrong heart and their "worship" will be not.
Especially for a new musician, they need to be mentored and guided and maintain a teachable spirit.
I have learned that some people are just not ready to receive counsel or instruction no matter how much it may benefit them. Hey, that's cool. Maybe they're maturing at a slower pace than others? And if that's the case then they can mature themselves off the praise team and let the Holy Spirit work some humility in them.
When they're ready to receive some constructive criticism and education, play for them a few examples of how a seasoned pro would handle a variety of songs. Show them how Paul Jackson Jr. and others like him can lay into a groove that compliments the bass and drums...even though he has the ability to lay waste to most any lead guitarist...he just knows when to add just a little in the way of a rhythm.
If the cat only responds to Prince then find a few examples of Prince holding down a simple rhythm.
"Purple Rain" might be a good example. Also, "What the Fuss" from Stevie Wonder's newest studio record features Prince laying down a really nice "in the pocket" pentatonic-based lick throughout the song, embellishing a little here and there but always returning to the familiar and very simple lick.
Remember that sometimes people front because they're scared or unfamiliar with what's being asked of them and rather than possibly embarrass themselves, they put up and try to act like a stubborn man...all the while, making themselves act a fool...so sometimes the teaching needs to be handled one-on-one and with carefully measured words that edify and not tear down.
It's at these times when you really know if someone is teachable or not.
If they're not, maybe it's time for them to take a holiday until they are willing to be.
