1. How much content do they have in DNN already?
2. How much of that is going to be tough to migrate out of there?
3. What kinds of customizations have already be done in DNN?
4. Why are they contemplating moving to a new l(a|n)(m|pg)(d|r|p) platform and who do they know to program in these new technologies? Is that you? It might make sense to move to a linux, nginx|apache, postgre, django|rails platform if they want to make a website that is very custom and needs to be fairly maleable.
It wasn't great. Developing modules was confusing and overly complex. The documentation was horrible. The data access stack was plain stupid if you follow the "correct" way. It's written in VB.Net and many of the examples were in VB.Net. (the VB.Net thing was a big reason I didn't continue)
If they're bringing you in to make a clean start with the best web frameworks available, then one of the Ruby/Python/PHP frameworks others mention would be better IMHO.
If the answer is somewhere in the middle, you might have a look at the Castle Project (http://www.castleproject.org/), which I've had some good results with in the past.