The Evenflo base has an adjustment on it to help with the right recline. You have to push in a button and then kind of pull down on the little foot. Assuming it's a newer model, there'll be a 1 2 or 3 that will show through the window. I'm not sure how slanted her seats are in the Corolla, but that should be enough to get it installed.
If she's wanting to install it in the middle, check the manual to see if middle latch is allowed or not. Chances are she'll need to install it with a seatbelt in the middle, and that could be what the problem is.
Go through the manual with her while you install it and you should be fine. I think Evenflo has manuals available online if you have the model # and DOM info.
As for difference between Keyfit and Evenflo, I can't really compare the 2 because we don't have the Keyfit up here. I believe the Keyfit minimum weight is 4lbs, so it will probably fit tiny babies better - as in preemies, but a full term baby is going to fit fine in an evenflo carrier. If her evenflo seat has the front adjust, I'd probably rather use a new evenflo seat than a used keyfit unless it's a really super super close friend.
For the length of time that babies are in a carrier, I don't know that it's worth getting into a debate since she already has the evenflo and doesn't want to use the keyfit. My suggestion would be to have her look at the keyfit, and have her buckle a doll into each and see how she likes adjusting the harness on both - and maybe try installing both in her car. There's nothing wrong with an evenflo carrier, they're just not as nice... If anything will change your friends mind, it will be the difference in ease of installation and use. And I'm still iffy on the used seat vs. new seat she already has. :twocents:
eta: I just re-read your post and realize the seat is coming from your SIL, so you have a pretty good idea of the history of the seat. I still wouldn't push it though. Regardless of what personal preferences are, the best seat is one that fits the child, fits the car and will be used correctly each and every time. If the evenflo carrier meets these criteria, there's really nothing wrong with it.