Time and timing are the tricks: opportunity and exposure, as well as speed -slow it down.
I'm a native English speaker, I also speak other languages, like Japanese and French and the list goes on, and now I'm learning Chinese. I get by OK in China, until that is I have to speed up the conversation, like when I'm in a taxi telling the driver to take a right or stop here and I blurt out Japanese. A similar thing happened years ago when I was in Mexico. While shopping in the markets or ordering food at a restuarant, where all is calm and relaxed, Spanish came easily, but when I had to think on my feet quickly, French came blurting out of my mouth. Hmm.
Try to get the student to slow down enough to take time to think about what s/he is saying. Timming and Time are the keys.
All the best.