1500 series should have 2 1/2" wide leafs. I think a 2500 series could have either 2 1/2 or 3" wide leafs, depending on GVWR. Match up the correct width 2500 springs and they should interchange.
It would be a good time to inspect the spring mounts and the shackles for rust and replace if necessary.
If they are OK, yes he should be able to install complete 2500 series springs. If he didn't break main leafs - the top leaf with the loops at each end - he could always just replace the broken leafs and add a few extra leafs to each side - this would avoid the hassle of dealing with the mount and shackle bolts.
If the spring change raises the truck more than an inch or so, longer shocks should be installed.
And a strong warning - even after "up-grading" to 2500 series rated springs, he will still be running on a 1500 series 9 1/4" rear axle, 5 lug wheels, and probably light truck tires at best.