As this individual the OP is speaking about is a Catholic one can in no way assume they are “ignorant”. Also the OP states that he does not know if the individual has gone to confession. As always, we can not make any sort of assumption as to the state of an individuals soul, so “admonish the sinner” does not work here either.
Now if you witness a Catholic commit a sin, you might say something but again, only mortal sin separates one from receiving the Eucharist and there really is no way for us to know that an individual committed a mortal sin as there are three conditions that must be met for it to happen.
*]object is grave matter
*]full knowledge
*]deliberate consent
In witnessing a sin you can only know number 1.
Yes we can teach Catholics what is necessary but that is done in the context of teaching. Talking with a friend is not the place for this to occur unless they ask you about it.