Okay, I know a fair bit about this because I've both had a section and scar dishenience (breakdown).
These days, caesarians are done on the low part of the pubic area (mine is actually on my pubic bone, prodigiously low, the midwives were stunned). They are also transverse (sideways). So generally you end up with a discreet smiley scar, rather than a classical (vertical) cut.
The reason for this is that the uterus has different sorts of muscles at the bottom as the top. The top has vertical muscles (imagine a hot air balloon made of long vertical strips - a bit like that) while at the bottom, the muscles run horizontally - (think ribbed condom stylee).
During labour the lower muscles pull upwards to expand the cervix and allow the baby out. (That's what your contractions are about) while the upper muscles massage and push the baby downwards and towards the cervix. It's really a wonder of engineering how it works so well together.
Anyway, caesarian wounds are stitched in layers these days - normally at least two. In some cases a drawstring technique is used which binds muscle fibres together is such a way that the area where the scar is is not all that flexible but is very strong.
Disadvantages to his are of course that it is bloody sore while it's healing and it can lead to a lumpier scar and numbness.
During late pregnancy any scar will thin and stretch - your skin reacts to it's normal state, you wouldn't expect to lose 20 pounds and still have a scar in exactly the same place would you?
With a caesarian you get bruising too which can cause extra swelling on the uterus so it doesn't go back so quick as with a normal delivery.
Scar tissue lacks collagen so it can't stretch so easily. This can lead to partial dishenience (scar breakdown). Where the scar entirely breaks down through all levels you can get uterine rupture (read: bad, bad thing). But it's very rare with recent surgeries because of the technique used.
If she has only had one CS and it's not within the last twelve months she's probably okay. The cut btw is not large, but the scar may be if she stretched it post partum (I did - gardening).
I tore some internal stitches after I fell pregnant the month after a laporoscopy - it bloody hurts I can tell you. But with internal stitches, no you can't see a thing from the outside, except you can get a bruise if the stitches are close to the skin because obv it'll bleed. Again these things heal themselves.
What your friend will probably be offered is a 'trial of labour' or 'trial of scar'. It's entirely up to her, but second caesarians are much more risky - in terms of death, hysterectomy etc. than VBACs so it may be worth her having a go. Chances are it will all be fine.
I have, as I said, scar dishenience (I'm 22 weeks) but it is normal to experience some changes in a cs scar during pregnancy. So I'm not particularly worried about it.
Hope that helps (bet you wish you hadn't asked now?)