第二个that依然引导定语从句，指的依然是this guy，语义上也就是this guy went on and ended up working on an oil rig somewhere。went on and ended up...表示后来怎么样了——这个男的后来在某个地方当了个石油钻井工。

我找到的英文版本是这样（少了个逗号，第二个that定从的先行词看作是guy + 第一个that定从，这种定语从句的结构称为双重定语从句）：

There was this guy that she was really madly in love with that went on and ended up working on an oil rig somewhere.