1995 Volkswagen clutch slipping

So my mom drives my old 1995 VW Jetta. It’s got over 160,000 miles on it and has had the clutch for about 90,000 of those. She took it to her mechanic because the clutch was slipping. He was in the process of replacing the clutch and found an oil leak in the transmission. Now the transmission is off being repaired, which she nor I was expecting needed done. Does this sound plausible? And, if so, will the transmission need rebuilt, or just new gasket(s)? Thanks for the feedback!

At that age and mileage it’s quite possible the transmission could be leaking.
This could be the reason the clutch was slipping. Once oil starts leaking onto a clutch disc it only takes a little bit to knock out a clutch. Engine oil leaking from a rear main seal on the engine can do the same thing and it would be a good idea to replace that rear main seal while the trans is out. It’s right there and very little is involved in changing it.

One would hate to put it back together and the seal start leaking the next week since this means all back apart again.
The trans should not need a rebuild if there was no noise involved with it.
Hope that helps.