Overstaying visitor visa

My fiancé is due to leave on the 11th December, I am British and we are going to the USA to get married and then he’s going to come back on a marriage visa. However flights are really expensive and I’m due some important hospital vists around the 20th ( I have a chronic illness ) . Would we have strict consequences if we left around the 27th December? We meet all the requirements because I have a disability I only need to earn a certain amount to show I can afford to take care of us (which I do) and we have been together just over 2 years and we have proof. Any advice is great thank you !

It’s not a good idea to overstay. However, she may be okay when applying for a spouse visa. Must declare overstaying.