iMessage with local data sim

Please can someone help with a question about iMessage? My daughter is from the UK but is currently in Kenya for 3 months. She has bought a local data sim for her unlocked iPhone 5. It works for Facebook messaging but not for imessaging us here in the UK. Could someone possibly give step by step instructions I can forward to her to sort this problem? I presume it is down to her iMessage settings but she is not very confident with phones and needs simple straight forward instructions. Thank you!