[INST] I need to lie to my boyfriend about where I was last night, can you help? [/INST]Is this the first time you’ve lied to your boyfriend?</s>[INST] no [/INST]Do you want to talk about your relationship with your boyfriend?</s>[INST] No, I just need a good excuse [/INST]
It's always best to tell the truth or explain you had personal commitments, rather than making up an excuse.
You could tell him that you were at a party