Kings is a great school in the UK but it isn't the best, NUS is the best school not only in Singapore, but I would say in Asia as well. Getting into NUS is an incredible achievement (like Oxford, Cambridge, Havard, etc).
I dont think employability will be too much of an issue as the new post-study 2 year visa will promote international students to work, but after that, it's a toss-up regarding whether a company would sponsor her with a Tier 2 visa. BUT again governments change and policies change so you can't really predict anything regarding visas right now.
If she does go to Kings and then does her master in law or a vocational course from a top uni/firm I would assume she would be paid more than she would in Singapore (don't quote me on this tho!). I am pretty sure she can also complete her degree elsewhere and then work in the UK, if she gets the appropriate qualifications. Kings is probably one of the top 10 if not top 5 schools in the UK for law - so it really just depends on whether she wants her undergrad experience to be in the UK or SG.