A very common position, if they are on the mortgage documents jointly they are probably jointly and severally liable, which means they are both liable to pay the full amount not just their 50% share. The mortgage provider will probably start court action for possession when the arrears build up, as has been said your Daughter will not be expected to move out quickly and court action will be required to evict her which may lead to action by bailiffs.
Some mortgage providers are not keen in the present market conditions to force a sale unless they can be sure they will recover all monies that are owed.
Your Daughter should visit a Solicitor as soon as possible; it will be a good idea for her to write down all her queries to ensure she does not forget something when with the Solicitor.