I'm not a Lawyer so my advise is just that. . . advice.
[b] #1. Is it legal to prohibit an employee from carrying a firearm when there is no federal, state or local law broken? [/b]
Unfortunately they can prohibit firearms (both employee's and patrons) However, its not illegal for you to carry there; its just against their company policy. Ie: you won't go to jail for it, just fired. Unless your state has a law prohibiting carry where signs are posted banning their presence, then you'd be FUBAR.
[b]#2. Can an employer prohibit firearms from being legally stored in a vehicle?[/b]
Yes it is legal to make a rule such as this, if you park on their property. BUT, who's gonna know? Are you or your friend going to let someone from the company search your car????? I'm sure as hell not going to, I'd make them fire me instead! Also, off-property parking they can't touch, no business can tell you what to do when off their property (except drugs, which you technically would be bringing on their property if you went to work with them in your blood/tissue).
[b]#3. Should the "friend" carry anyway and risk losing the job?[/b]
I might not carry, but the car would surely have a weapon in it. Thats what I did when I worked at Winn Dixie for 9 years. They have a similar policy and I had no trouble because I didn't tell anyone (Learned my lesson after a Glock was stolen from my car while at work)
[b]#4. If said job was lost, is there room for a lawsuit to be brought against the company?[/b]
There is always room to sue someone. You can sue your wife for not washing the laundry on time if you wish. Whether you would win or not is a totally different story. You probably wouldn't have a leg to stand-on in your stated case. Now if your store gets robbed and you get injured or killed, and you didn't have a gun on you because of company policy, I'd bet attorneys would be biting at the bit trying to get your case against the Company (even if it was your widow they'd have to swoon over).
The real question is which is more important, having a job or being relatively secure in your ability to defend yourself?
Just my .02 YMMV
Steven L.