Call her. Check in and let her know you hope it all goes well. No need to mention prayer.

Cross my fingers?

Say something along the lines of "you're in my thoughts". That's what I do.

"Prayers work if they were true from the heart" = positive thinking and truly wishing that someone get well would work..
According to the law of attraction ?

i have to say, not in direct response to your question but in response to a response that for some reason has no reply button on it, that when i have a problem and i communicate it, i really hate when people say they're sure everything will be fine. how do they know? i mean, if it's obviously a case of my insecurity about something, sure, that's reassuring, but if it's a medical issue, or a financial issue, no, there is no reason to say it will all be fine, and yet people say it, meaning to be reassuring. even strangers will do this. i find it horrifying.

g

i have been asked for prayers, not by family but by friends. i always say "i don't pray, but i will be thinking about you, and i hope for the very best."

g

I give them medical advice.

I experience that all the time. People often ask you to pray for a whole lot of different things. I just interpret the situation as them wanting to hear words of solace and assurance. I usually say something along the lines of: "Sure! I'm sure everything will be fine and if there's anything I can do, I'm here for you". I think it's best if you make sense of this message in terms of the bigger picture and not take it at face value.