Let us remind ourselves that a foil is a character that is used to highlight the characteristics of another character by comparison. Therefore, the humour of a character can be exaggerated by placing him next to an extremely boring or unhumorous character. You are right in identifying that Benvolio at certain parts of the play acts as a foil to Mercutio, and this scenario of wit is certainly one of the traits of Mercutio that is emphasised through this technique. One scene where we can see this in operation in Act II scene 4, when Mercutio and Romeo engage in a series of puns and a battle of wits. In the verbal duel that follows, interestingly Benvolio does not at all participate, except to make a final comment. His silence and his obvious lack of ability to engage in a verbal duel only makes Mercutio's quick tongue and wit all the more obvious.