Visa overstayers in the UK

Hi

I have a question regarding overstaying a working holiday makers visa;

A friend of mine has overstayed her visa by 1.5 years; she is leaving the country for good from Gatwick this April.....

What are the chances of her being caught out at the airport? and what will happen to her?? she is really stressing about this; she knows she shouldnt have done it,

she is leaving quite soon; any information will be greatly appreciated.

Thanks