By the way, in American English, "to blow" is also slang for "to suck / not be any good". For instance, "That movie blows." = "That movie sucks." = "That movie isn't any good."
The sentence "She blows." can mean two things: 1. She sucks d*ck. (like in your sentence) and 2. She isn't any good. / She is a terrible person. Hence it is ambiguous unless you know the context. Your sentence definitely means #1 since it has the direct object "that guy" and it is in the past tense. haha