i don't know how to respond to that

When someone either genuinely does not know how to respond to something or is trying to avoid the topic of conversation. Often used to make situations awkward, and can be used to end conversations.

friend: my girlfriend just cheated on me
me: I'm so sorry to hear that
friend: I wish I could get over this in two seconds, ugh I hate life.
me: I don't know how to respond to that