How to approach student in lab with very bad English?
As an adviser or mentor it can be interesting to learn how the person you are advising/mentoring is feeling. Based on the response, you might gain a clearer idea on how to approach finding a solution. It seems that the individual in question understands written English quite well given you mention you have chosen to communicate via email. I would suggest sending them a message asking them how they are feeling with the program so far and with their work in the lab, and what are some challenges he has faced or is facing. You could ask whether he feels his English proficiency has been problematic so far, and add a couple of other items to like working with the equipment and colleagues, for example. This opens a space for conversation that is minimally face-threatening to the individual and invites them to seek advice and guidance to help their better functioning in the lab.