Some ppl might disagree with my personal opinion, but here it
goes...
I would recommend a sportbike. Not for its speed (I would be crazy
to encourage a beginner to speed), but for its stability and ease
of use. A sportbike is inherently designed to be stable at high
speed. And if it's stable at high speed, it'll also be stable at
low speed. It also has high maneuverability. It can easily take
bends, corners, u-turns, straights, well... you name it. It doesn't
handle bumps and potholes as well as a scram does, but luckily
Singapore roads don't have a lot of them.
Another good point is sportbikes are also inherently designed to be
light without compromising their stability. So flicking one from
side to side on a twisty road doesn't require much effort. And for
2B bikes, the power delivery is not as intimidating as their bigger
counterparts, shouldn't be a problem for beginners.
But there're downsides to sportbikes. They're not the best bikes
for pillions. They tend to consume more petrol because of higher
performance. And riding one at not-so-high speed on a long distance
could be taxing on your wrists, shoulders, back, and bums. Which
brings us to my next recommendation: a sport-tourer.
A sport-tourer offers the same quality as a sportbike does, without
the body ache. The riding posture is more relaxed, so it's suitable
for long distance riding. And sport-tourers are friendlier to
pillions.
Another alternative is a cub, which is designed for easy riding.
Hence, they're easy to ride 
The downside is they're not as stable as the previous bikes I
mentioned. They can be swayed by a strong gush of wind, and light
impact can knock them down. These're not major drawbacks, though,
as long as the rider takes good precaution when riding.
Having said all that... I could have been biased, because I don't
have enuff experience with scramblers, cruisers, scooters, and
other types of bike to make good comparison. So in the end I guess
your GF herself must make the decision, which type of bike she
likes the most, and suits her riding style the most. Did you say
she was going to get the bike next week? It's been more than a week
since your 1st posting, so it might be too late for me to babble in
this thread... oh well... 
