What kind of relation can one have with persons who formerly used to commit such unlawful acts as drinking?

Answer

The criterion is the present behavior of people. So if they have repented of what they used to do, they should be treated presently like other believers. As for someone who commits ḥarām acts at present, it is obligatory to deter him from it through forbidding evil. To prevent him from committing a ḥarām act, it is obligatory to avoid his company and break relationship with him if it is the only solution.