I think like all things relationship that its simply a matter of the people involved. Is there risk, sure if the person is already poor at relationships. Ideally in a perfect world this would never happen, but it does.
Not every relationship is built on a perfect structure. I guess its a matter of degrees. How messed up is the existing relationship opening up?
And I agree with sage to a degree. A new relationship can offer new energy, perspective etc, as long as the incoming partner isn't going to cause more pain. Support and love and all that jazz would always be a good addition. As long as all parties view it as such