They can spay a female that is in heat, but they usually charge more for it.
One of my Yorkies went into heat just before I had her spayed and she was fine. I had been in the hospital and my adult son was watching her. I came home and knew the second I that I saw her that she was in heat. Not sure how I knew - instinct, I guess. I waited so I wouldn't have to pay the extra $100.00 they wanted.
She will be just fine. I have a Maltese, too and she is getting spayed on Monday... I'm so hoping she doesn't go into heat before then....