How come relationship went so far without forewarning?
Complicated relations like this may not be good idea.
The daughter needs to be advised to control her emotions. A good guy but bad or potentially bad family is not a good combination.
How will you keep in touch with her in days or years to come if she marries?
As mentioned earlier it was level of trust between us, she always kept me informed, always told me that he is a good friend but (it is one way traffic) she does not share same feeling for him.
About two years ago, she have told him clearly that there is no possibility any marriage due to family circumstance advised him to move on, but agree to keep normal friendship with him.
From then onward i did not follow up on the issue (although there is always doubt about this on back of my mind)