...His mom took away the gun for now...
I'm curious how Mom got the gun away from a legal adult in legal possession? Apparently she still has some control over him. I'm guessing it is because he still lives with her (probably rent-free).
Does your daughter live with you? If so - that should give you some control over her behavior and choice of friends.
How old is your daughter? If she is a minor you may have some leverage with statutory rape charges.
This guy sounds like real bad news waiting to happen. I agree with others here that the gun is not really the issue. Fists, knives, sticks, cars, rocks - all provide effective tools for assault if that is the intent.
Warning: thread drift...
I have a sister-in-law WAY past the age of knowing better. Her long-time live-in BF has been doing this kind of intimidation thing for years. It doesn't get better - it DOES get worse. He started out following her and threatening or even assaulting people she was seen with. She does openly cheat on him, so their whole mess is definately a two-way street.
Everyone in the family has allowed themselves to get drawn into their drama at one point or another - usually when she SWEARS she is really leaving him THIS time. The last time he kicked in the side of her car I bought her some pepper spray (she wouldn't have a gun) and told her she was probably next.
A few days later they were out getting s-faced drunk. He got pissed because she let someone buy her a drink, so she decided to leave early. He followed a little while later and left the road at an estimated 100 mph. He was found the next day in the ditch still alive somehow. So guess what? She is now devoting her life to caring for her brain-damaged BF who can no longer work.
Back on-thread:
I guess my point for the OP is:
1.) This doesn't have anything to do with the easy accessability of firearms.
2.) Do what you can to end this thing before it becomes a bad habit.