Easy sonatas
The three easiest Beethoven sonatas are Op. 49 no. 2, Op. 49 no. 1 and Op. 79. If she can manage 49/2, she will probably be able to manage 49/1 (slightly more difficult) and 79 (again slightly more difficult and longer). Both 49/1 and 79 are far superior musically than 49/2, which is really something Beethoven probably wrote as an exercise for a pupil. (It was published without his knowledge by his brother, and Beethoven fumed for a while about it – incidentally, the menuet theme had already been published as part of the septet in Eflat). I suggest she hears the three of them (there are countless recordings of it) so she can choose the one that she likes the best (49/1 has a most appealing rondo)
Haydn also has a number of sonatas at her level (most of a similar style to 49/2). John McCabe has recorded all of them for Decca, and so has Jeno Jando for Naxos (great value!): Hob XVI/8 in G major; Hob XVI/7 in C major; Hob XVI/9 in F major; Hob XVI/G1 in G major; Hob XVI/11 in G major; Hob XVI/10 in C major; Hob XVI/D1 in D major; Hob XVI/4(5) in D major; Hob XVI/1 in C major; Hob XVI/3 in C major; Hob XVI/13 in D major are all delightful, and all equal or below grade 5 ABRSM (I have highlighted my personal favourites).
Mozart’s three easiest sonatas are in order of difficulty: K 545 (around grade 5/6 ABRSM) in C major, K 282 in Eb major and K 283 in G major. If she already plays the Turkish rondo, she might want to go ahead and learn the other movements of K 331, but they are more difficult than the rondo, and more difficult than the other three sonatas suggested here. (Several recordings, I like Mitsuko Uchida and Andras Schiff)
Scarlatti composed almost 600 sonatas encompassing all levels of difficulty. They are all in one single movement and much shorter than what we usually understand by the term sonata. Two very beautiful and lyrical ones that are around grade 5 ABRSM are K69 in F minor and K 213 in D minor (they sound much more difficult than they are). (several recordings most notably Mikhail Pletnev – Naxos is recording the full set).
Two obscure composers, Christian Latrobe (3 sonatas op. 3) and Fanciszek Lessel (3 sonatas op. 2) are also around grade 5/6 and the style is similar to Haydn. I know of no recordings, and would be interested to be informed.
I hope this helps.
Best wishes,
Bernhard.