You'd have to destroy their soul gems, allowing their souls to return to their bodies... and costing them everything they've worked for.

[His expression is one of deep sympathy. How did he work that much out, from the information she gave him?]

There is a process in alchemy called the magnum opus - the great work. The first stage of it is called Nigredo - the blackening or decomposition. That sounds remarkably like what your friends are experiencing. The second stage... is called Albedo, and it involves the purification of the material.

I may be able to recreate the Albedo stage in such a way that your friends can use it to purify their gems.