Well, presumably, this violinist is a competent composer and is intimately familiar with Mozart's style, likely through playing a bunch of his violin works. So I'm gonna guess your question is not "how does one imitate Mozart's style well." If it is, I'd push you to read Robert Gjerdingen's Music in the Galant Style, which thoroughly examines a variety of melodic / harmonic formulas that made up the vocabulary of eighteenth century music.
If our violinist/composer has a competent imitation of Mozart that he wants to pass off as his own, he should do some of the following:
1.) Claim it's an early piece, from when Mozart was like 10 years old (so like the 1760s). If people hear oddities in it or if they think it's just not up to the calibre we tend to associate with Mozart, this strategy lets said people attribute these weird things to random musings of an immature composer just starting to come to grips with his own style, rather than the true reason (that you momentarily failed to imitate Mozart's style well). Another reason why you want to do this is that Mozart had a personal catalogue of his own works that he kept as a mature composer, and if you attribute it to a mature Mozart, you'd have to explain how this fully composed piece somehow didn't make it into the catalogue.
2.) Assuming that this is an orchestral piece (like a concerto), arrange it for piano and attach your name as an editor. In general, people understand that an arrangement for piano has to make certain compromises and adjustments to make the orchestral texture work on a new medium. And especially in the nineteenth century, editors were often very generous about what they did when adapting or arranging another's work. So with your real name as editor, you give another outlet for people to say "oh, this is weird! The editor probably made an intervention here to adapt Mozart's score to modern tastes, I bet in Mozart's original score he did it like so..."
Basically, your strategy is to give a cheritable audience as many opportunities as possible to attribute something weird in the piece to some part of the process that lead from Mozart's manuscript to the version that they see in front of them, rather than as evidence that the piece isn't truly in Mozart's style at all and is really an imitation of it.
Someone who did all of this was Marius Casadesus, who forged a Mozart concerto which he termed the "Adélaïde" concerto. He and his brother were quite adept at forgeries, and clearly understood eighteenth century style well. So much so that it was considered authentic by noted Mozart scholars such as Friedrich Blume and was given an official entry in the Mozart categorization numbering system (as K. Anh. 294a).
For more info, see a detailed examination of 3 case studies by Erin Smith: "Mozart, Pergolesi, Handel?: A Study of Three Forgeries." As well as the prefaces and critical reports for the critical edition of Mozart's works (the Neue Mozart Ausgabe), especially Series X (Supplement) Band 29 (Works of Doubtful Authenticity). Handily, the NMA is available for free online, but unfortunately, all the reports are in German.