You dont do anything... I have friends that are liberal. It doesnt make them less capable of being a good friend. Friendship is a investment in many things and as long as you get a return on that investment, its not a wasted venture.
I have several friends who are anti gun.. I respect that position to the degree that I do not carry to their home or in the cars but other than that, I tell them to shut up about it. I have had no problems with it nor have I lost any friends over it.
"I believe that the right of the citizen to keep and bear arms must not be infringed if liberty in America is to survive." - Ronald Reagan