Thanks for your question.
I'm afraid that Just Answer prohibits the attaching of documents/private correspondence between customers and experts, it is more for general advice.
Guarantees are fairly common, particularly where the landlord has had their fingers burnt by bad tenants in the past or where agents use them as a matter of course in student lets. The extent of your liability under the guarantee depends on how wide the wording is. I note you say it is unlimited, does it refer just to your daughter and her tenancy, or the tenancy as a whole?
If it refers to the tenancy and your daughter and her flatmates are all signing one document which refers to them being 'jointly and severally liable' then this will mean that you are guaranteeing the rent (and any other sums due under the agreement) of all of the tenants together. Of course, the other parents of your daughter's friends are probably being asked to sign a guarantee and you should check with them to make sure there are other guarantors to limit your exposure.
If the guarantee just refers to her and her tenancy agreement and each flatmate has their own tenancy agreement then you are only exposed to the costs your daughter incurs.
Ultimately, it comes down to trust but you should check for joint and several liability and get in contact with the other guarantors to check your are not going to be the only left without a chair.
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if all named as jointly severally -