There's a bit of a legal loophole here (I speak from experience now).
If your friend signed as guarantor in the presence of two witnesses (one a solicitor) who signed to witness your friend's signature as guarantor, then yes she is legally bound to pay any rent owing.
However, as often happens, if the owner of the business or their representing solicitor simply posted the form out to your friend for it to be signed in private and not in the presence of witnesses who would attest to the reliability of your friend's signature, she could claim that she did not sign the guarantor's form and that the signature therein is false, and therefore she cannot be held responsible for the rental debts.
This happened to me. Tenants in a house I owned owed six months rent when they finally moved out.
When the guarantor was contacted to claim the unpaid rent, the guarantor claimed the signature on the form was not his. My solicitor had posted the form out to him for him to sign at home, so there were no legal witnesses to his signature. The guarantor therefore did not pay the outstanding rent.
I was advised that the legal costs of trying to prove that the signature was indeed the guarantor's would far exceed the amount of rent outstanding, so I cut my losses and left it.