Irtidad or apostasy is to reject Islam (its theology and tenets) after
having accepted it. The term does not apply to one who is a Muslim but
does not practice its rites like salat, fasting, hijab etc.
There are two types of apostate, one who is born into the religion and
thereafter rejects it (fitri) and one who converts to Islam and then later
turns away from it (milli).
If a woman turns apostate she must be imprisoned. For a man, the
punishment for one who does this willingly, and is baligh and sane, is
death in the first case, and death after 3 days, if he does not recant, in
the second case.
However, this is not a simple matter to judge and great caution must be
taken so that no oppression ensues from mislabelling individuals. About
apostasy (irtidad) Ayat Sistani says in his Q&A:
"An apostate is a person who changes his religion - if a person denies the
Prophet (S) and his message then he is an apostate. " and he also says
"A Murtad must be identified as truly having rejected the faith and it is
not for people to go about labelling others as apostate and declaring that
they should be killed."
In the West, the best way in my humble opinion to deal with both cases is
to counsel and resolve doubts if possible. Inshallah in time they will be
guided aright again