What should I do if a witness asks me what to say?
Don't try to tell a witness what to say. The witness has an obligation to tell the truth, and any attempt to induce the witness either to withhold their testimony or to "disappear" from the jurisdiction can amount to Witness Tampering, a felony. I know of a case where a defendant called the alleged victim and left a simple voicemail stating "let's see if we can work this out without the need to go to court". The defendant was convicted of Witness Tampering and got a year in jail. It doesn't matter if the witness approached you or you approached them. Don't take the chance, stay away from witnesses and let your lawyer talk to them.
For more information regarding your question, contact a criminal defense lawyer at Perez & Perez Law, PLLC.