If you go to work and people bully you there, then can you sue and complain to the police?
Police enforce criminal law, not civil law and what is going on at work is about civil law. You might want to suggest he not write his complaint with a plan to be vengeful to the employees he has a conflict because that "appears" very much like the same poor treatment that he is making the complaint about, hence petty retribution. Your friend can sue the perpetrators but must prove they are the originators of the rumors (very tough to do) and that the were offered as a matter of fact lie rather than a suspicion that others concluded on their own or just repeating something they heard (also very tough to do) and then prove how he was damaged as it equates to money. (often difficult too). There is a lot of risk in doing so, because if your friend fails to prove his case, he stands to lose his job for exactly what he is accusing others of doing.
If your 'friend' wishes to sue, that is up to them... not sure what you think your friend can sue for.....the police are not interested in work place bullying, that is an internal employer issue your 'friend' needed to report when your 'friend' was working there
I was in a similar situation and I even overhead people at the office saying that I spread my legs and that I am gay or smell like a wet dog I reported that conduct to the company and the managers who were part of the bullying were fired Another person at our company received $100,000 in compensation for bullying after being reinstated in his position that he was bullied out of