this person I know is now having their pay withheld for unspecified reasons. I'm guessing there are laws against this? How well protected are foreigners against unscrupulous companies?
So this person you know had his/her EP rejected but is travelling for work and is supposed to get paid but the salary is held... Well, not sure what to recommend here... First your friend is careless to have started work without EP (not to mention he has 0 insurance coverage if something happens while travelling...). There are indeed laws against unscrupulous employers, but I think the one facing the most risks here is your friend... I would say good chance to get deported... Not to mention, how can you friend live in Singapore without EP, for hp, housing, bank, ...?