And remember, when he writes in English, he's not writing in his native language.
[WS:] From my experience, it is an advantage not a handicap. Proficient ESL speakers typically have two things working for them - they usually have more exposure to standard language than to vernacular which makes their expressions sound more standard (i.e. what is generally considered better), and they benefit from being multilingual which expands their assortment of forms of expression.
Wojtek