You say the business is closing. One assumes this is a sudden change of events and the business has declared itself insolvent?
If so, she is in a redundancy situation. In that event, the people that wind-up the business (administrators or whoever have a responsibility to the redundant workers and either they or the Government's compensation scheme will pay out her monies due.
That is the good news. The bad news is that this will not happen by next Thursday so she should make plans as best she can to conserve money to tide her over. Others including Ethel can probably better advise how best to do this - places to go for small term 'social' funds if she is completely without.
I am guessing that it might take say 4 weeks at best for this to resolve itself. Legally she is entitled to money for her notice period plus statutory redundancy.
More info here.
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10026695
Your question about some weekly-paid workers having been paid for this week and she hasn't doesn't help, I'm afraid - her contract is for being paid monthly. She will get her money - its the timing that is a problem.