The good news is, life expectancy is increasing for transplant recipients. Ten years is average, but that data is a bit old now. That said, the likelihood she will live 30-40 years is very low -- but we keep advancing health care. Just a generation ago, nobody lived past 30 with cystic fibrosis. Now that's routine.
There is another significant aspect though: You mentioned children. She can't have children -- not while she is on anti-rejection drugs. She has to stay on anti-rejection drugs for the rest of her life or her immune system will destroy her new heart. Those drugs are highly toxic to a developing fetus and cause fatal birth defects. She can't have kids on the drugs and she can't come off the drugs or her heart will die.
A transplant is not a cure for anything -- it is trading one set of problems for another. It is trading the guarantee that you will die of heart failure for a lifetime of complicated treatment and other risks. She will always be at high risk of infection. She will always be at risk for organ rejection. Everyone who gets old, declines with age so you have to add that in there as well. For you, you need to decide what if any of that is a deal-breaker. She doesn't have a choice; she needs to live with the management of a complex disorder. You need to decide how much of that you want to take on as well.
Working on my master's in nursing