Youch!
First thing you do is SOAK it in conditioner. I mean, empty half a bottle of conditioner on it, and gently work it in. If you ahve a conditioner that you know is esp. 'slippery' - use that. Let it sit for a while - at least an hour, under a shower cap.
Find the WIDEST tooth comb you can.
After the hour is over, section her hair into eight sections - four front, four back. Start fingercombing her hair in sections- don't rinse the conditioner out, at all!! Finger comb though it, and gently untangle the knots as you find them - start from the bottom of the section, and go through the whole thing. Then, get the wide toothed comb, and do the same thing. Then, if you have one, get a medium tooth comb, and to the same thing.
If you need to, add more conditioner.
Do this to each section - it'll take a while, but it should detangle her hair.
Good luck!