For my earlier post, what I mean is Serangoon Central has MRT
station labelled Serangoon and in my opinion, it is a rather
prominent landmark. Besides, the bus stop just outside Serangoon
Central also has a flyover.
Plus, Serangoon is going to be an interchange for the years to
come. So, it will be a more prominent landmark.
Anyway, looking from your explanation, he boarded the bus 80 from
somewhere in Upper Serangoon at 4 plus, reach keppel or
vivo/harbourfront at 1715 and witnessed the act. So, there is a
significant time lapse here.
However, after reading from the description, my intrepretation is
as soon as he boarded the bus at Serangoon Central, he witnessed
the act.