This is a strange little thing, but it sounds awkward to stay “step forward” with the verb “stop.” I don’t know if there’s a rule against saying “a step forward by stopping” but it sounds strange. Instead, you would say, “He took a step forward and stopped drinking Coke a week ago” or “He took a step forward by cutting Coke out from his diet.”

Also, it’s not considered polite to say “a very fat friend” … we would typically say, “I have a friend who needs to lose some weight” or “an overweight friend”. Even with those words, you’d need to be careful!

I have a very fat friend and he needs to lose weight. He took a step forward stopping drink coke a week ago.