Merge (or maybe rebase) question

Another how best to do this.

- I have my project all nicely in git with all the different releases
and versions in there going back years. Happy :-)

- I have a customer who has heaviliy modified an oldish version of the
software.

- I have identified the version that they modified and created a
branch in my git repo based on that version (i.e. what they would have
started with before modifying it)

- Their changes where applied to a simple download - no version
control, no nothing.

I now need to update them to the latest version of the software - can git help me here or is it all a disaster?

As usual, any advice much appreciated.

PS. To make matters worse - their original is full of expanced CVS $Id:$ tags so it will look to git like every single file has changed. Damn you cvs!!!