My own stepmom is younger than me. She and Dad started dating when she was 19 and Dad was in his 40s. She is a wonderful, Christian woman and the best thing that ever happened to him. My mom is wife #1 and lasted 4 years, #2 lasted 18 years, #3 less than a year, but #4 lasted almost 30 years until he died [February 18, 2012]. I am forever grateful, as a Christian, that Stepmom led my dad to have a relationship with Christ. He was a much better person for having loved Stepmom. Like your friends, her parents did not approve and are probably happy Dad is dead.
My advice: don't be judgmental, don't take sides [you are not required to approve nor disapprove], just be their friend. They will need a friend like you during the rough emotional seas ahead.