In case you can not read chinese characters on your PC, I also created
an image (birthday.jpg) for you to download from my site.
http://www.priviet.com/birthday.jpg

Note: In cantonese, it is different ways to address mother's and father's father.

P.S. According to Chinese tradition, when greeting a senior's birthday,
you should greet him in Chinese way, thus I prepared for you the second
version in this image. All these characters are simplified ones. Feel free to write to me if you need traditional ones. [email protected]

If Grandpa is her mother's father, the translation will be,"祝外公生日快乐。我爱你！”；if Grandpa is father's father, the translation will be,"祝爷爷生日快乐。我爱你！”。

This answer above is for her father's father.If you want for her mother's father,then she has to use “亞公”in place of “亞爺”.Because her grandfather speaks cantonese and vietnameze,I think it's better to use cantonese and traditional Chinese.