There's something weird with the combination of verbs here.
"They should read a lot, which helps them improve their language."
If this is some kind of suggestion (should) the second part should express the result for what is being suggested. In this case it should read:
"They should read a lot, which might/may/will help them improve their language."
But if the second part is the current truth, I mean if the second part is correct, then the firt part should be a guess and the line should read:
"They must read a lot, which helps them improve their language."
oh yes I understand what you mean.
I used the modal "should" to refer to the same meaning as "must" so it is not a suggestion but a recommendation (necessity too) as it is in the original sentence.