Ok. Try this and see if it works.
Sources/BoardIndex.php
Find
WHERE $user_info[query_see_board]
AND b.ID_CAT = c.ID_CAT
AND b.childLevel <= 1
ORDER BY c.catOrder, b.childLevel, b.boardOrder", __FILE__, __LINE__);
Replace
WHERE $user_info[query_see_board] || b.ID_BOARD = '{boardid}'
AND b.ID_CAT = c.ID_CAT
AND b.childLevel <= 1
ORDER BY c.catOrder, b.childLevel, b.boardOrder", __FILE__, __LINE__);
Just replace {boardid} with the ID number of the board you want them to see. I think that will work, but if it doesn't, let me know.