What is a fair Scrabble handicap
I play Scrabble with kids all the time, I think the same concepts would work nicely with an ESL opponent. To me handicapping by points (opponent starts with 150 or 200 points or whatever) does nothing to make the game more fun for either player. What does work is allowing my young opponent all the time they need for each turn - I get timed. They get all of the good cheat sheets - I don't (that one is fair even even if I'm playing an adult native English speaker with less Scrabble experience). And if the gap is really big - especially if Scrabble is being used as an educational tool - allowing my opponent timed use of the dictionary or OSPD while figuring their play. For the last one, consider limiting the time the opponent gets to use the dictionary more and more as the gap narrows.

BTW, using timing the stronger player as a handicap only works if that player draws tiles at the beginning of their turn instead of the end of their last one.