Lending an android phone to someone for temporary use, consequence?

[AC Question]

Need to lend my android phone to someone for temporarily while he shops for a new phone. I use this phone when I travel overseas, so I have a foreign country SIM card, basic apps, mostly used are Whatsapp and mail (can delete mailbox and add later).
What will happen to the data in my phone with my SIM, when the person inserts his SIM, it will read his local carrier (ATT), will it mess up my contacts, Whatsapp access later when I put my SIM back?
Anything else to watch. Thanks.

 

[Rukbat]

What will happen to the data in my phone with my SIM, when the person inserts his SIM, it will read his local carrier (ATT)

Assuming it's not SIM-locked and it has the required radios for AT&T towers.

will it mess up my contacts, Whatsapp access later when I put my SIM back?

No, unless he manually messes them up.

Anything else to watch. Thanks.

He'll have access to ALL the data on the phone. AT&T has a phone for about $50. If he uses it for less than 14 days, he can return it and cancel the account.