Another let down friend over money again!?

I recommend that you have them pay for their own tickets in advance, Then after they come to the event, then you reimburse them.  If they don't show or they cancel on you, You DON'T reimburse them.  

maybe you can have your surgery where you are and then you wont have all those expenses

Some airlines do offer a cancellation service, minus an administration fee. Check with your airline. Some airlines will not allow a change of name once the ticket has been purchased. Check that out too. If they do allow a change of user and do not offer refunds, then do your best to find another person willing to take the journey. Unfortunately, they may not be happy to pay the transfer fees as they are doing you a favor, not the other way around. But it's a small price to pay to have the company and someone to be there for you when you have the surgery. Be sure the friend you asked in the first place is definitely canceling before you do anything else otherwise you could end up losing that friend on a technicality. Once you have established that your friend is definitely not going you should choose someone else quickly and hold your breath. Sadly, people, today cannot be relied upon. Not even your best friends.

If you need a companion with you to take you home and such (some surgeries require it), you could check with the hospital staff or whoever is doing the procedure, and ensure your place will not be lost if your friend backs out. Find out if they have other ways to help like letting you stay in hospital longer or setting you up with an aide during that time in hospital. You might have to pay more, but sometimes it is covered by insurance.

I am sorry about your friend, but she has not backed out yet, and still might not. Ask her.