Admission is case-by-case and the principal has the
discretionary power to accept or decline an application. Your
cousin (with her parents or guardian) will need to request an
appointment with the principal for an interview.
However, Malaysian SPM is not usually accepted as a substitute
for 'O' level results. Which means that it is likely your
cousin will first have to spend 2010 as an 'O' level private
candidate, prove she can score all As at 'O' levels, then apply in
2011.
If she's going to take 'O' levels as a private candidate 2010,
she might as well self-study (eg. with tuition) 'A' levels and
take it as a private candidate in 2010 as well. If she does not
meet the requirements (eg. nationality, age, etc) for private
'A' levels (ie. H2), or even local 'O' levels, another option
is the British 'A' levels and 'O' levels examination, which
you can apply at the British Council.