Phone advice UK to SEA

Hiya, moronic sister leaving for 18months travel, about 6months in South East Asia and 1 year Australia in literally 4 DAYS

Has now realised she needs to figure out her phone. Any advice when it comes to getting a sim, pay as you go, signal etc

She wants to keep her number to use WhatsApp. Worried she'll have to leave the sim at home so it can stay active. Would she need a new phone, new WhatsApp, new sim? I'm very much clueless but would like her gone so need advice.

Thanks in advance