Since this system is owned by a friend I assume that he would help you analyze this opportunity to upgrade. Do you and your friend both have CD players hooked up to your systems? If so you can start with a simple comparison that will answer a lot of questions. Note: When I say Turntable I mean the entire system including tonearm and cartridge.
Pick a vinyl record and the corresponding CD title from your collection that is good demo material. If you lack either the CD or the record, buy it. Compare the CD and the vinyl on your system and note the differences - frequency response, detail, soundstaging, etc. Take the CD and vinyl to your friend's house and do the same. Does his TT blow away the CD? What are the differences there? Does the CD sound amazing too?
This test can be valuable for a couple of reasons: 1) CD players are much more uniform in SQ than turntables so they can provide a good reference, and 2) If you hear substantial differences between your friend's TT and CD player it will help you analyze what you are hearing and what appeals to you.
This is easier than @tomic601 's suggestion of lugging your TT to your friend's house but I heartily endorse his idea too. Before you contemplate spending tens of thousands of dollars it would make sense to do some due diligence. Besides, geeking out with your audio buddy is a great way to spend an afternoon.