The guy can constantly remember the former girlfriend, compare her to the real girl, try as often as possible and closer communicate with it – it occurs only in case for him these relations are not finished yet. He cannot emotionally release it.
A lot of things depend on how the guy and his ex-girlfriend left – if in the relations there was an innuendo, or process was followed by scandals and hysterics, then, perhaps, feelings of the guy did not cool down yet. By quiet parting, as a rule, everything manages to be found out, to understand and realize. But no matter how everything occurred, from it is only more painful to you. Try to define the attitude towards the guy – whether there will be enough at you sincere forces to struggle with rather strong competitor whether you want to become for the guy the important person, the companion. Do not hope to take the place of the former girlfriend, you need to create such conditions under which he will appreciate you not less. To fight against ghosts quite difficult, but in the presence of desire to understand the friend everything can be overcome. If you communicate in one company, then it is possible to try to become the friend of his ex-girlfriend – it will help you to exclude it from the list of enemies, at the same time to stop all attempts to revenge from her party and to gain trust of the guy. If the guy continues to meet the ex-girlfriend, then hint him that it is unpleasant to you. It is not necessary to arrange interrogations, to bother with inquiries and to try to find out details of their novel – you will only stir up former feelings and distance go from yourself. Act more cleverly – show the behavior as you are upset, it is possible to try to stop the relations for some time. Do not accompany temporary parting with ultimatums – the choice it will be obvious not in your advantage. The most constructive option is to stop the unpromising relations. If the guy really loves the ex-girlfriend, then it means that he perceives you only as a temporary object for a consolation. You do not want to become pale replacement of the former beloved because the person even can not bear daily comparison with the steadiest mentality. Have with the guy a heart-to-heart talk and offer parting, designating the reason – let he will understand why everything occurs quite so. Do not agree to the relations "three together" at all – the guy can suggest you to wait, help to forget to him the past, but it is only a trick. Respect yourself and do not spend time for the person who cannot deal with own life.