I suspect the right to have someone removed from the property depends on who is legally considered the tenant and that would be found on the lease. If your boyfriend signed the lease along with his mother, they would be both legal tenants of this apartment. Some landlords might require that all adults sign the lease because it makes all people living in the apartment legally liable if rent is not paid.
If his name is not on the lease, technically, all guests are his mother's guests. In that case, she can ask you to leave and she can ask the police to remove you from the property. However, this is close to a domestic issue. You're not exactly trespassing when her son, who also lives there, offers you to come in. It's likelier they will just tell you to leave.
On a more personal note, I would suggest that you carefully weigh what you gain from making an enemy. On one hand, you can't always let people threaten you and push you around. Your goals are important and they need to be acknowledged. On the other hand, annoying people come with consequences. Some people are vengeful and they will find ways to hurt you.
What I am saying is there might be a more sensible way to solve this problem than calling her bluff and risking to deal with the police, even if the interaction with the police might be benign. Maybe you're asking the wrong question, in other words.