Any clue what this flag/decoration is?

Just taking a SWAG here, but the front part is the shape of an Army, Navy or Air Force guidon. USMC guidon flags are rectangular. Like MSG (Join to see) said, I too assume the '83 is for a year (1883? 1983?)

I discounted Navy and Air Force guidons as they are a uniform color (the Air Force does have some white/blue ones).

The color scheme is not the one for cavalry units (which is red over white, not the other way around) and white over red is associated with 1st Army. However, the Army's heraldic regulation* doesn't show any 'white over red' color scheme for the service.

I have seen bad knock-offs on eBay* that showed a reversed color scheme for the red and white fields.

Between 1836 and 1841, cavalry unit guidons usually showed the regiment on the bottom in red lettering on a white field and a U.S. in white lettering on a red field.

It almost seems that someone made the guidon from a description and flipped it for the 12th cavalry. Only one problem - the 12th cavalry wasn't stood up until 1901 and by that time the guidance had dropped the U.S. on the guidon (it was only done between 1836 and 1841).
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* AR 840-10 - https://history.army.mil/html/forcestruc/docs/AR840-10.pdf
* 7th Cav Guidon (later 1830s) - https://cdn.shopify.com/s/files/1/0109/5535/8271/files/1836guidon.jpg?v= [login to see]
* eBay bad reproduction - https://www.ebay.com/itm/b0833v-US-Army-4th-Cavalry-Guide-on-1870s-thru-today-/ [login to see] 38

Not sure what the '83 represents. Perhaps the year? But, the other part being the 12 over the US with the white over red signifies this possibly to be the 12th US Cavalry. But, normally it's Red over White.

You need to repost this with the tags Marine and Camp Lejune or this post will just disappear.

I know I'm coming in late, and didn't scroll through all the comments, but this is an Americas' Cup Lapel Pin. Not a military decoration at all, for all that it looks like one. It can be Google searched as "Americas' Cup lapel Pin with '83"

It's most likely a tie clip. Ask your friend if his Dad was a Scout Master in 1983, Troop 12. Maybe worn with a suit and tie when not in his Scout Master uniform.

I am attempting to attach a document from the Marine Corp University. The 12 Marine Division has a storied history. Since your father was in the Corp this might be a good place to start. The only current 12th is an Arrillery Regiment in Okinawa I believe.

https://www.usmcu.edu/Portals/218/Brief%20History%20of%20the%2012th%20Marines.pdf

Try The Institute of Heraldry Website that would a great place to start
https://tioh.army.mil/

Looking at the pin, I doubt it was 1883. I would say a guidon flag? But I really don't know much about them.

Looks like an Army Cav pendant, and there in may be the issue. Some units had their own unique signature items (some Army Cav units went so far as boots, stetson's, neckerchiefs and such). Have you tried Google Lens?

Could it be that dad swapped insignia with some army guy as a souvenir ?

From my research, which is admittedly limited since I'm in Washington State, that appears to be an Army Cavalry tie clip from the 12th Cavalry Regiment out of Fort Hood. The unit was established in 1901 so that clip would be from 1983.