Of course...though you do have to be prepared for the possibility of her being refused entry (the only people guaranteed entry are US citizens....even non USCs with visas and green cards can still be refused). Therefore in order to reduce the possibility of this happening she would need to make sure she has and arm full of ties back to her home country to prove that she is going to return (ie, a letter from an employer stating when she is due back at work, mortgage papers, rental/lease agreements etc...though this should never be volunteered and only shown should the officer at the POE asks for it). She may also have a strike against her as having a K1 in the works shows future immigrant intent, therefore she should have strong ties back to Mexico. Though at the end of the day, it will be entirely up to the POE officer that inspects her attempted entry to the US......many people have been able to visit their fiances with no problems while others have been denied and sent back home.