do you think that former lover can still be friends after all?

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i have this friend who already have a new boyfriend and her ex also moved-on. the former guy built relationship with my friends family, so though they were not at present, the guy still use to visit the family not the girl anymore and at different house just near...when i and our friends heard of this scenario, we gave different comments such as that it is not right anymore for the former guy to visit to whatever purpose he have. my friend and his present actually had quarrel after that....for what the real reason we don't know...but most of us thought then that is not really appropriate anymore....my other friend concluded, so we mean former lover can't be friend anymore? maybe yes...but most of the time no...and way too risky...it might shake the present....what is your point of view?