There wouldn't be much point in people seeking guarantees if they knew that the guarantor could pull out of the arrangement!
Sorry, but you're wise to be concerned about the possibility of having to pay your daughter's (ex-)boyfriend's debts. However you might wish to consider entering into a time-limited guarantee. (i.e. signing a document which says that you'll guarantee the payments for, say, the first year of the tenancy). If such a guarantee would be acceptable to the landlord, it could give your daughter and her boyfriend time to prove that they're decent tenants, while still minimising any losses which you might incur.
Chris