Feet on sofas

I have a friend who, every time she visits, makes herself comfortable and puts her feet up on my sofa.

I was brought up that you don't put your feet up on a sofa, let alone someone else's sofa. Her feet are sometimes dirty, or if she stays a night or 2, and she's had her bath, she puts her feet up in her dirty slippers! Her last visit, the bottom of her slippers had lovely brown stains and skid marks. I was very dismayed but didn't say anything.

(In my own case, I might put my clean feet on the sofa but usually tucked into my dressing gown - but not when she is visiting me - I tuck my legs up and leave my feet hanging off the sofa).

My friend has always said to me, when I visit, that I can put my feet up (on her sofa) and I always decline saying that I've been brought up not to do so as it is considered rude. I might relent, if my feet are newly washed and clean, removing my slippers but pull my legs up on the sofa and let my feet hang off the edge.

AIBU to feel disappointed that my friend is being disrespectful by putting her feet up on my sofa?
How can I gently tell her not to do so without offending her/what would you do or say?