Parents want to apply FLR(FP)

[Evicra]

Dear members
i hope you all are in best of health and everything.

my first cousin and her husband arrived in UK about 8 years ago .
1. she came here on student visa with her spouse
2. couldn't get extension as college licence was revoked. and her fee was never refunded.
3.they have a uk born son who is now 7 1/2 years old and doing very well at KS2

they are thinking to apply for FLR(FP)

i request you seniors to please provide best possible solution if there is any so they get LTR

if FLR(FP) is the solution then what documents they should provide in order to support their claim

their only son is doing very well at KS1.
both husband and wife are on signing list.