Simon
I'm sure you're traumatised about your friend's plight. But there
is a right way and a wrong way to be civic conscious - you will be
penalised if you go overboard.
If the thief died because of a heart attack and not due to any
manhandling by your friend, then a court of law will/should find
that there was no causal link between the thief's death and your
friend's actions.
You may have found 1000 hits on 'third degree murder', but you
should be looking to see if any one of those hits have anything to
do with SIngapore law. Third degree murder is more mostly an
american concept, which is not even present in UK law.
You could reach a conclusion that helping anyone in need is wrong,
heck, there'll be many posters who will agree with you (this also
explains why we will always get people standing around and looking
instead of helping) - but I'm sure if you had a relative being
attacked; you'd want someone to help.
If you act with good conscience and with prudence, you will never
be in trouble with the law.