Has she done the system update on the S4? Because my wife got tired of her s3 around the time I got the Note 4, so I gave her my old s4. She was complaining about it a bit, but not as bad as the s3. Then, she got the lollipop update and got excited about it like a new phone again.
Anyway, if she likes the latest system update to the s4 better than how it was previously, that's more in line with what the s6 will be like, though, obviously, the s6 has some better hardware and fancier features baked in.
If she's not a tech nut like many of us are here, the s6 really probably is the way to go, considering she's had 2 Samsung Galaxy phones in a row so far. Many people don't like Samsung's touchwiz interface, but after you've become accustomed to it's added features, it's hard to give up the few that are actually useful.
No matter what, I can't agree more with the idea to take her in to the store and let her play with a few different devices for a bit and see if she stumbles onto something she really likes about one over the other.
