When Summer says "Guidance came to me in the form of a greasy-haired lunatic", she is referring to Cletus, a neighbor boy who has befriended her and Ob and, in his inimitable way, is helping them deal with the grief of losing May (Chapter 6).
Summer doesn't like Cletus very much at first, and she is not very nice to him either. Cletus is inarguably eccentric; he is an incorrigible collector - of potato chip bags, buttons, wrapping paper, and finally, pictures, of every sort and subject. Summer thinks Cletus is "a flat-out lunatic", and doesn't much appreciate his over-the-top friendliness (Chapter 3), the ridiculous-looking hat with the fake fur earflaps that he likes to wear (Chapter 5), and his greasy, stringy black hair (Chapter 4). Cletus starts hanging around with Ob and Summer soon after May dies, and in his unique way, he distracts them from the depths of their grief. Ob especially is quite taken by the boy, and Summer's dislike of Cletus is increased when she realizes that sometimes, Ob tells Cletus things that he doesn't even tell her.
Beneath his off-beat demeanor, however, Cletus is astutely observant, and surprisingly wise, and he becomes for Ob "the perfect consoler, because he listen(s) to every word Ob (says) and (keeps) his big mouth shut" when the situation calls for it. Little by little, as Summer begins to understand that Ob is not the only one consumed by his grief, and that she has deep issues herself that must be resolved, she turns to Cletus for help and guidance (Chapter 5). Just when she feels the most lost, Summer realizes that, unlikely though it may seem, guidance is available to her through Cletus, and, "desperate...(she passes) him the torch, hoping he can lead (her and Ob) out of (the) infernal darkness" of their loss (Chapter 6).