Yes if you dont want to be friends anymore then do it unless he is ok with it then u can still be friends and then every thing is going to be just fine
They can be if you don't get jealous. If someone has offended you by dating your ex, you are free to end your friendship with that person. However, you might also prefer to continue your friendship....
just ask him out. if your ex gets mad, don't worry about it. your ex will have to get over it soon. but if your ex doesn't get over it, then break up with his friend because it could lead to trouble.