From a medical point of view, the current research does support having a ceasear after significant 3rd/4th degree tearing to prevent further bowel injury.
This is generally accepted practice now, although of course women are able to refuse and go vaginal, and defiantely it won't be a problem for some, you just don't know if you are going to be the one that is ok or not.
They aren't scare mongering- it is evidence based practice as far as I know ( might be wrong here) and a midwife is able to discuss this as it is probably policy at the hospital to suggest it.
Has she seen a physio after the last birth who specialises in pelvic floors? I would be discussing with someone like that too- they see women after birth and know all the ins and outs.

Personally I would probably go the LUSCS, but it is her choice and she can do her research into preventing tears etc, she will have a lot of scarring in the area which will have an impact.

She should also have a growth scan to estimate the weight- if it is assessed to be bigger that may sway her decision. Induction might be considered too.

I hope everything goes well either way x