They can file charges if they want but it is up to the DA if he/she wants to prosecute. When it goes to the grand jury they must meet to burden of proof to go forward with the case. If the police believe the accuser's story they will most likely get a search warrant to search your friends...
It looks like you qualify for an expungement:
http://www.uslegalforms.com/lawdigest/expungement-criminal-records-law.php/OH/OH-EXP.htm
Who is eligible for an expungement?
You are eligible if you meet all of the following six conditions:
1) The conviction you are trying to expunge is...
Of coarse they can deny you a job for any given reason. But you were honest with them that you were not convicted so it shouldn't even show up on your background check. If you want to try to get a few more replys check go here
It may have not even made it to the grand jury yet. Unless there is more proof the DA will probably drop the charges. If not hire an attorney or ask for a public defender to see what other evidence they may have against him. Your attorney will know how to handle this.
Warrants never expire. It's best if you take care of it so you don't have it hanging over your head for the rest of your life. How do you know there is a warrant?