1) Yes. Most of these companies & business are legal under
ACRA registration.
2) Yes. If they choose to do it without violating peace of
anyone like your neighbours other than you. These days locating
one's address with next-kin info and the rest is a piece-of-cake.
For example; just look at how your SINGPASS can link to all your
infos. In fact, any agency willing to pay the required fees (set by
government agencies that administer this such service) to search
for your data and this is legal.
3) Under law any form of agreement between two parties, if both
agreed, is legitimate. Unless you file for dispute for not agreeing
to the sms agreement, you are forever liable.
4) Report? Again, you should read all the threads here before
this one. You could file report if there's any violation made by
the agency like causing hurt or distress etc. But still won't free
you from the debt. I believed your friend did it in order to remind
you that you are not going to escape easily.
I believed your debt is below $10k and if the amount is $10k and
above, your friend can file petition against you for
bankruptcy.
Do note that if your friend is all out to get you, then you
should prepare for the worst. Your friend too can turn to law firm
instead and exercise other legal options like Writ of Summons, Writ
of Seizure or freeze, deduct and seize your monthly pay via
Garnishee Order. Think about it.
Hope this helps. ;)