The phrase "be foil to" implies opposition or hindrance to something or someone. Some antonyms that can be used for this phrase include "assist," "abet," "support," and "facilitate." These words all imply a positive contribution to the success or progress of an endeavor, rather than obstructing it. Other antonyms for "be foil to" can include "encourage," "promote," "enable," and "foster." These words suggest a willingness to aid or facilitate the achievement of a goal or objective, rather than obstructing or hindering it. By using these antonyms, we can shift the focus from negativity to positivity and help promote collaboration and success.