My app gelding's mane has gotten like that. I sprayed a lot of conditioner on it, making sure I got the front and back of it. I let it sit for a bit and then I used my fingers to start untangling everything. when I got to about 90% done, I used a bristle from a brush to get out the smaller, more difficult tangles.
When I was done, I sprayed his mane again with the conditioner and brushed it.
He has gotten them again, but I have just left them in because I know it's going to happen again.
If you have the owner's permission, I would trim it a bit.