If his name is Tony Smith, and you know him well as Tony, then
If his name is Mr. Tony, then as per your original.
Dear Mr. Tony
I would like to inform you that yesterday evening, Friday 26th February, an incident involving actual physical assault occurred in the store. At approximately 7:00 PM, two customers in an argument engaged in verbal abuse, before physically attacking each other. I summoned the city police to deal with the situation. They interviewed both customers..
and, for you bosses information, what was the outcome? eg
before accompanying them to the police station.
I am happy to report that no damage to the store was incurred.