Douglas Adams already worked out the situation:

Lallafa wrote about a girl who had left him and precisely what he thought about that. Long after his death his poems were found and wondered over. News of them spread like morning sunlight. For centuries they illuminated and watered the lives of many people whose lives might otherwise have been darker and dryer. Then, shortly after the invention of time travel, some major correcting fluid manufacturers wondered whether his poems might have been better still if he had had access to say a few words to that effect. They traveled the time waves; they found him. They explained the situation – with some difficulty – to him, and did indeed persuade him. In fact they persuaded him to such effect that he became extremely rich at their hands, and the girl about whom he was otherwise destined to write with such precision never got around to leaving him, and in fact they moved out of the forest to a rather nice pad in town and he frequently commuted to the future to do talk shows, on which he sparkled wittily.

Lallafa never got around to writing the poems, of course, which was a problem, but an easily solved one. The manufacturers of correcting fluid simply packed him off for a week somewhere with a copy of a later edition of his book and stacks of dried habra leaves to copy them out onto, making the odd deliberate mistake and correction on the way.