I see this as two separate but related messes. if I may put it that way. Firstly, he has his photos spread all over the place and he wants to consolidate them onto one large hard drive now that he has a new computer. That's a good thing to do regardless of the image editing application he has. It's hard to keep track of where the original master files are when they are spread in numerous drives, some of which could fail at any time. It is also easier to back-up one's photos if they are all stored on one drive that gets backed up, daily if necessary. I back-up my main working drive onto an external bootable drive with Carbon Copy Cloner every night, so I know I have much of my computer in at least two separate accessible, bootable places. When he consolidates all his photos into one drive, he should decide on an appropriate file structure for the photos that makes sense to him, so he knows exactly how to find his master photos regardless of the image editing application. Remember, Lightroom does not store photos. It stores links to the photos. That is what the catalog is - links to photos, along with their metadata. If he has his edits of his raw files in separate XMP sidecars, those sidecars need to be kept with the master photos when he does the hard drive consolidation.
Now, having consolidated everything into one properly organized hard drive, he could create a new catalog in Lightroom by importing into Lightroom everything he wants to be in Lightroom folder by folder in a structure that resembles how his hard drive is set-up, and make a separately named "Collection" for each imported folder before moving onto the next one. This is the "clean start" approach, which will take time. But he'll have a fresh coherent set-up between Lightroom and his hard drive. Some experienced Lightroom users don't care about this coherence because they use keywords etc. to find everything they want regardless of where it is. I think doing both can be useful.
The alternative approach with Lightroom will be to do it all through Lightroom, which will involve a two step operation: (1) merging all his previous catalogs into a new catalog by following Lr's (and Andrew's) instructions for merging catalogs, but (2) as he will have re-organized his hard-drive outside Lightroom, he will then need, from within Lightroom to relink the master files to the new catalog by doing a succession of "find missing photos" following Lr instructions for doing this seek and replace for these links. It allows one to find one photo in a folder and then relink all the photos in the same folder with one instruction.
Finally, contrary to the first approach above, instead of re-organizing his hard drives from outside Lightroom, he can do that reorganization (moving master photos from one drive to another from within Lightroom, which means Lightroom will know where everything is moved without step (2) just above. He would still want to have one merged catalog reflecting that whole reorganization.