I think in my head - the use of 5 for the whole chord as written can be done, but you're essentially having to lift the 5 finger multiple times in succession - lifting off the single note to get to the chord, and lifting again to get to the single note above before trying to finish the final chord in the slur.
Since the top note is doubled at the bottom of the chord, I think it's best to retain the smooth slur and omit the doubled note at the bottom. She struggles rolling with even the 4 as well, but I think for an exam - the control of the sound is what is mostly important.
Thanks for all the discussion points from everyone. It's nice to see what other people think... I mean, I have smallish hands for a guy but my student has a very narrow (really narrow) stretch.
The choice outside of the grade books is quite large if you check the syllabus.
Yes, I know there is a whole manual list of pieces, but I don't think she has the time and energy going through them all.