i agree with ignoring him, or better, smiling and being friendly etc - he is clearly doing this to get to her - and its working - if she shows she couldn't care less, his behaviour loses it power, and he will likely stop.
i would stare back a bit, pretend to whisper to colleagues while looking at him and start laughing, shake your head pityingly, make overly friendly comments 'how is your tea sir, i hope its all that you hoped it would be'
and generally smirk at him etc
he will soon realise he's wasting his time.
she cannot do much because going into a public cafe and drinking tea is not harassment, and the staring is really just her opinion - he could say shes paranoid.