Change your profile name to CallMeStupid..........................you can't. So unless you have a homie-sexual thing for Robert Plant, leave it alone.
Cut the BS Danny............that you even see Robert Plant in her face tells me that you enjoy R. Plant a little too much. Ask her to marry you, finding a woman who looks like your idol is a once in a lifetime thing.
Can you give, at leat, one reason why you would want to make an issue of this in the first place? The fact that you know it could be offensive is admitting that what you *think* is not the best idea in the world. And if you know this isn't the best idea in the world, then why on earth do you give it space to ask the question when you already know the element of offence which this might cause?
The answer would be that you *don't* tell her anything ... because you already know it would, most likely, offend her. In short, you have all-but answered your own question here.
TELL HER SHE LOOKS LIKE ROBERT PLANT..........................YOU HAVE NO CONTROL OVER HOW SHE FEELS.
I WRITE IN ALL CAPS DUE TO MY FAILING EYESIGHT.
Tell her to take it as a compliment,if the intentions are good.
What kind of plant is he? A factory? Cannibis-aka marijuana? A rosebush? Or a cherry tree? And why does she need to know? How would you like me to tell you you look like Austin Powers?
Tell her she looks like Robert Plant, but that someone would probably still wanna give her every inch of their love.