Reading the Wikipedia page on Allen Dulles, there is a mention of him having served as a member of the Board for United Fruit in the section regarding the coup in Guatemala. What is written in the article is as follows

Eduardo Galeano described Dulles as a former member of the United Fruit Company's Board of Directors. However, in a detailed examination of the connections between the United Fruit Company and the Eisenhower Administration, Immerman makes no mention of Dulles being part of the United Fruit Company's Board, although he does note that Sullivan & Cromwell had represented the company.

Additionally, the Wikipedia page on United Fruit has a section about their reputation stating:

The integrity of John Foster Dulles' "anti-Communist" motives has been discredited, since Dulles and his law firm of Sullivan & Cromwell negotiated the land giveaways to the United Fruit Company in Guatemala and Honduras. John Foster Dulles' brother, Allen Dulles, also did legal work for United Fruit and sat on its board of directors.

I haven't been able to find any record of him serving on the board of directors, so any answer would be greatly appreciated!

edit: Apologies for misspelling Allen in the comments, I'm more used to using Allan.