Sam wrote:

Route wise, my suggestion is Oakleigh Park to KX, walk to KX
Thameslink, and then FCC to East Croydon.
OR
Northern Line to London Bridge, then FCC or Southern to East Croydon.

Oakleigh Park does not have direct trains to Kings Cross in the peaks.

An alternative which she might consider is Oakleigh Park to Moorgate, then
Metropolitan Line to Farringdon, then Thameslink to East Croydon - she would
be guaranteed a seat after Moorgate.

Also Oakleigh Park to Highbury & Islington, cross-platform interchange for
(crush-loaded) Victoria Line to Kings Cross, then short walk underground for
Thameslink to East Croydon.