They are hardy turtles, but they need a lotttttt of space. Adult males need to be in 50-60 gallon tank; full size female about 100 gallons.
Turtles also live a very long time. Water turtles can live up to 60 years, and land turtles up to 100 years. Unless your friend is absolutely certain they want this commitment, I hope he doesn't. And I kinda think if he was serious he'd be checking it out himself?
I hope you've talked him out of it. I love my slider, but I think she's a lot of work. Not hard to take care of, just a lot of work.