He acted like my friend but was actually screwing me the whole time.

screwing 是动词 screw 的现在分词形式。screw 本意是"用螺丝固定"，在俚语中引申为"欺骗、占便宜"的意思，常带有不怀好意的含义。这里用进行时表示持续性的欺骗行为。