Jesus ate with the worst sinners of His day. He loved them, but He never condoned their actions. He forgave the sexually immoral, but then told them to "go and sin no more."
Personally, I have no idea what I would do in a scenario like that. The trick is to share the truth in love, to show them that you are genuinely concerned for them, but I simply don't know if I have what it takes to walk that fine line.