If I were a future father-in-law, with a similar medical condition, I would not want my son's (or daughter's) wedding -- certainly one of the most important days of his/her life -- be postponed on my account, regardless of how uncomfotable it may be for me to attend their wedding on their planned date. Rather, I would want to see them wed now, while I am still with them.

I have a relative who passed away a few years ago from a late-diagnosis stage 4 pancreatic cancer. She had a boyfriend for a long time and had plans to get married at some point in the future. When they learned about the inevitable, they made arrangements and got married on her death bed in the hospital. She passed away a few days later.

I would have done the same if I were the boyfriend.