I don't worry too much about the butterfly effect because it goes both ways. Sure one stoplight here makes a guy die, but another stoplight over there saves a guys life.
But in this case, when it's a clear "either I put off changing my oil and washing my car, or this 250 lb senior citizen with gout tries going on a hike", it's a lot more clear.
Perhap he shouldn't have gone with her. Perhaps your gf shouldn't have let him go with her. Perhaps you face marriage into a family that has hereditary bad judgment!
Changing your oil and washing your car may be the luckiest decision you've made in your life!
I once did not marry a (beautiful) daughter of a family with an overweight heart-diseased patron. Turned out to be one of the best accidents of life for me. Saw her years later and she was almost as big as her Daddy was when he had his second heart bypass! Glad I missed that "iceberg in the night"!