Filtering issues after changing sand

My brother recently changed the sand in his filter. After he put it back together, he noticed that the it wasn't filtering very well. So he pulled all of the sand out again to check the laterals. Everything looked fine. Re-filled again and is having the same problems.

Observations:
- When you backwash, there is dirty water coming from the filter so I know it is partially working.

- Whenever he vacuums or adds DE, it looks like that majority of it flows straight back into the pool.

- After dealing with an algae bloom, he ran the filter non-stop for about 2 weeks trying to filter out the dead algae. It never did filter out and clear up the water. So he ended up turning the filter off for a couple of days and all of the particulates fell to the bottom of the pool.

Any ideas of what to do / what to check?

I'm no expert and will gladly defer to the more experienced posters but....at various times over the last three years I have replaced the sand in my filter, replaced the filter itself and also replaced the pump...it sounds like the filter is operating incorrectly and so I would say it may be possible he's got the lines reversed...I did that when I changed out my pump. Came here for advice and then changed the order of connection on the intake and output of the filter and voila...problem solved. Filter was sucking in and pumping out but at a very reduced rate compared to "normal". My mistake was not paying enough attention to the assembly before taking the hoses off. Now Ive got them all labled so it's idiot proof. Good luck.

Why did he feel the need to change the sand in the first place? In general, normal use, sand shouldn't ever need replacement. So why? If the filter was ALREADY malfunctioning I'm guessing he blamed the wrong potential cause--the sand.
Did he use actual filter sand or sandbox, sand-blaster or mason's sand? If it's not filter sand you will get problems.
Did he use the correct amount for his filter? Too little or too much can create problems.
Did he check the hole-size on the laterals to ensure they weren't too big, that there weren't any cracks?
Is he sure his multi-valve is functioning correctly? A bad m-valve can cause all kinds of problems.