Liszt double concerto!
Just kidding. Seriously, try sight-reading the Brahms or Beethoven sonatas together and see what happens. Or look at some later pieces, the impressionistic Faure sonata or (my favorite) the Franck (4th mov. is gorgeous). Brahms 3 is really cool, the second is very underrated. I don't care for the first. Kreutzer is the best known Beethoven sonata, but they are all very good (Beethoven isn't known for writing mediocre sonatas [op. 49 excepted]). Or if you need something simpler to get the feel of accompanying, do some baroque stuff (Handel wrote many violin-keyboard things).
You can even do a violin concerto (reduced part) for fun... it's useful practice and some arrangements work quite nicely with piano reduction (mozarts especially). Extremely useful skill to accompany concerto soloists; and your friend most likely would really appreciate it.
One rather short intermediate-level piece came to mind - Brahms' 'sonatensatz' (sonata movement) in c minor, small-sized show piece if you've never accompanied before.
If you've never accompanied a violinist before, perhaps a few Words of Warning: violinists are very finicky musicians, they get really overexcited about some strange things like 'staying in tempo', it's some violinist eccentricity. Oh yeah, and never, ever suggest that they are out of tune. You won't live to regret it.
-Rach3