Yes, infidelity is the end of a relationship, because there is no way to have a relationship that is built on any kind of foundation after that. If a person cheats, there is no way to trust that the person can be an equal partner of a relationship if the person cannot be trusted when no one is looking.
Infidelity isn't necessarily the end of a relationship. People make mistakes. Infidelity is a fairly huge one, but time, forgiveness and healing can move things forward in a positive way. It may take years for someone to get over the infidelity, but it can happen. Jesus forgives us for everything, so why not each other when it comes to making one mistake? If someone fails to learn their lesson and cheats again, then you have a problem.