I like to start off as friends with any potential partner, so I introduce them to Lucas that way. Personally I like to 'get it out of the way' early on, before any proper relationship happens. It's just easier because I wouldn't carry on with it if I didn't believe they could have a good relationship with Lucas.
Originally I told FOB, wanting to be completely open and honest. Unfortunately he didn't show me the same respect so I don't bother anymore.