Is there any way to unlock an iphone without knowing the passcode

No.

For some iOS versions this is theoretically possible, but it's not really a risk. Folks that are capable of this are expensive, have some link to law enforcement or academia. This is not something that part-time hackers can do. But seriously, just 'fess up. Life is too short, and you're better off just telling her the truth.

ellenyuang wrote:

someone took my phone and threatens to get it unlocked by some professional or someone so she can check all my messages, just wondering if this was possible

If you have a 6S, no. The FBI paid an expert to hack one iPhone 5. It cost around $1 million. The way they did it won't work for a 6S, however.