How very scary for you and your friend.
You are not the first potential surrogate to wonder whether her IPs were making the right decision. I can only say that any of us can die in a freak accident any day. We have no idea when our children may need to be raised by a single parent for any number of reasons.
I think, personally, it is not up to me to decide whether my IPs will be worthy parents (unless it was obvious they were having children only for the purpose of selling them to the circus or something). Not everyone feels this way, and obviously you need to feel comfortable with your own decision.
Some points that stick out to me -in no particular order:
you don't YET know how long she has; she may have decades
you don't know YET how much treatment may be required
what if your friend could live comfortably long enough to see her child be brought into the world?
what if your friend's husband only want a piece of his wife to cherish after she's gone and their baby is the only key to his peace?
is their family supportive of them using a surrogate?
what would you and your husband say if the husband was trying to adopt a baby as a single dad and asked if you'd serve as a character reference?
have you done any of the medical or psychological testing required to be a surrogate?
do you meet all the legal requirements in your country to be a surrogate?
In the end, no one can make this decision for you. You're the one (and your family) who has to live with the outcome of your choice.