Yes it is very easy. Just make suer not to cancel her T-Mobile account, it needs to be active for the port process to work and transfer her number to AT&T. Also you should double-check that her number is portable and eligible to port out. Most should but it is good to be prepared.
http://www.wireless.att.com/cell-phone-service/transfer-your-number/
Make sure that she has her T-Mobile account information handy when you are ready to port her number.
Dave