Yes, my boys had it (discovered at 16 weeks, I think). Please direct her immediately (if she's not already found it) to the TTTS Foundation (www.tttsfoundation.org)
. They will overnight mail a package to her (and can mail one ot her doctor too if s/he doesn't see many cases). The good news is such late detection without major issues probably means it's a mild case--she must insist on close monitoring though.
It's an extremely stressful and horrible thing to go through. The good thing for her is that her babies are already viable. For me the worst part was all the pressure to be prepared to do an umbilical cord ligation (basically a selective reduction), and the day to day threat of death before they even got to the stage of viability. Your friend won't have to deal with all of that.
But please, please direct her to the TTTS foundation. There's a lot of crappy info out there, but that is probably the best source of info--and the people there are kind and will even put you in touch with a BTDT mom. I also personally corresponded with Dr. De Lia, he also was a good personal resource. But at this stage in the game it's going to be about monitoring the twins' health and taking them if it looks like things are deterioriating rapidly.
THe earlier you're diagnosed/noticeable, the worse it tends to be. As I said, ours was diagnosed as just a hair less severe than what was needed to qualify for the laser surgery (at that time they were using worst case scenario guidelines because it was still deemed 'experimental')--which means it was pretty bad. Yet I went to 36 weeks and only had them then because of MY health issues--not TTTS.
Dylan (my donor twin) had some issues that were similar to what IUGR babies experience, but you'd never know it now.
Edited to add: Dylan was normally 3-4 weeks behind Tom until week 32 when suddenly he started to slowly catch up. (VERY weird and unusual). I think that's pretty par for the course for moderate TTTS.