I'm not the DE expert but I know the screens can be cleaned if they are not damaged. Pool stores will do that for a fee, but they soak them in muriatic acid to clean them. You may not need to replace them, but you may. If you replace the filter, you almost certainly need to replace the multivalve to get one compatible with your pool.

At that time you CAN choose to stay with a DE filter or switch to sand or cartridge. That's up to you.

If water is going in his skimmer and coming out dirty from what appears to be his low drain....something is probably plumbed wrong. Who puts a return so low in a pool? Of course it is POSSIBLE--there's no physical reason I know of not to put a return low.

But if it is a low return, then simply properly cleaning the filter of that goo and re-introducing DE may fix it.

Just some ideas.