NEED HELP

Ok guys I need your help in pointing my brother-in-law in the right direction. He had replaced his gas block with a low profile one, then replaced the fore-end with magpul furniture. On top of that he mounted a rail attachment which put his flip up sight TOO HIGH. What can he do to rectify this problem? Would a gas block ( with a rail ) be in the correct sight-line for a flip up rear and/or optics?
Any thoughts?

Thanks in advance
Rkfulgor1

A pic would help.

Is the rail section on the handguard level with the rail on the top of the upper?
Are the sights the same height?

Easiest fix go with a flip front block / sight combo.
http://www.opticsplanet.com/yankee-h...-yhm-9394.html

OR...........
shit can the sights, buy a 1-4 and call it good.

Cheek riser?