The brace on a friends Rattler is extremely hard to fold once in the deployed position. He has to wedge a screwdriver under the pivot point to get the locking lug to release and allow folding.
Is there something that can be trimmed or polished to make this an easier task or perhaps a lighter compression spring that seats the detent? He is not handy at all and came to me for advice.
I have no personal experience with these pistols.

Find a competent gunsmith they can fix it

Not to sound like a jerk, but is he releasing it fully before trying to fold it?  The “detent” is a big square groove and it’s held in place by a lot of spring pressure. This makes it rock solid when open.  

In the folded position, there is an angle on the detent so you can just swing it open, but to close it you have to press on the top of the receiver with your thumb, lift the arm out of the groove then swing it closed. Once released, they usually pivot very easily. The thumb on top, fingers around the brace arm may help so he doesn’t need to use a screwdriver.