I hope your friend sorts it out Champagne. She should get out if there really is a problem, but she will still be liable for rent up until the end of the tenancy (another 3 months?).
I know some of the other replies are made in good faith, but I have to contradict them. It would be VERY unusual for an initial term of an AST (tenancy agreement) to have any sort of get out clause for the tenant. If it continues after this initial period (usually 6 months) then yes, it would be more common and certainly if the tenancy is periodic. But it would just make no sense to have a clause such as this where the tenant could just walk away in the initial term.
I would advise her to tell the agent or the landlord the situation and tell them she's getting out, but to offer to pay half of the rent for the remainder of the term - possibly 3 months. Legally, this wont put her in the clear as she will be jointly and severally liable for the whole rent, as will her partner, but if she does this then the agent/landlord will most likely chase her partner for the remainder. Just running off and hiding, as was suggested, wont work. It's very easy to track people down and its actually quite cheap to take them to court for rent owed. So, be upfront about the situation with the agent/landlord. If the agent wont listen, go directly to the landlord - legally you have to be supplied with their name and address if you ask the agent.
Technically you dont have to give notice to vacate at the end of the fixed tern, but its common practice to do so. She should do so now to reinforce the point that she's getting out.
And, yes, contact the police if she feels it warrants it.