Is the 1st Marine Division Patch an authorized Army combat patch?

WO1 (Join to see) are you aware of any units that received written permission under 19-17 a. (d) (3)?
[Unless otherwise approved by this regulation, Soldiers attached or under operational control (OPCON) to other
services are not authorized to wear their patches as their SSI–FWTS without written approval from DCS, G–1 or a desig-
nated representative.]
That doesn't mean other periods haven't been authorized by the DCS G1, after 2017. Authorizations between the 2014 version and the 2017 version should have been reflected in the 2017 version, as part of the change.
My company was assigned to 1 MARDIV during the authorized period. The 1MARDIV SGM asked me if my Soldiers would be able to wear the patch. I explained why we couldn't, and provided him the wording from the then current AR 670-1, and a draft proposed change, at hos request. He was adamant that he wanted to limit the time period to the period reflected in both changes. It wasn't the Army that wanted Soldiers to wear it, it began with him.
Now, the USMC may have changed their mind about limiting its wear, and the DCS, G1 may or may not seek input from the USMC before authorizing an Army unit to wear their 1MARDIV patch, but all of that is beyond the scope of my knowledge. I'm only aware of how it began.
To correct my response to the original question posed: Unless the Soldier was present during the period authorized by AR 670-1, or has written documentation from the DCS, G1 authorizing its wear, the wearing of the 1MARDIV patch as FWT SSI isn't authorized.
I'll add that if the Soldier was wearing it because he deployed with 1MARDIV as a Marine, it definitely isn't authorized.

Sort of like that nasty rumor about wearing a CIB if you were a MARINE and awarded a CAR. The whole Combat Patch/FWTS-SSI has been clarified since 2008 because of the patch hunters. Unless you were assigned during the authorized period you can NOT WEAR any MARINE patches and you also can not wear the patch if you were a MARINE now assigned to the Army.

WO1 (Join to see) - Chief if you deploy as an ARMY COMPANY SIZE element you wear your SSI as your FWTS-SSI that was established many years ago.

Howdy. I was 1 A.D in Ramadi 2006. I was on a C.O.L.T that had a Marine k9 team with us at all times. Afterwards I was sent to 1st infantry. I would often wear my 1st MEF patch. I even met General Peter Pace wearing it. He shook my hand and gave me his coin including SEAC William Gainey.

Thank you, 1SG, for this very thorough and comprehensive response. I would say that, based on this, the soldier I encountered was not authorized to wear the 1st Marine Division patch as his combat patch.

There is authorization, and then there is enforcement. I knew a former Marine infantryman who wore the 1st Marine Division combat patch on his BDUs and Class A’s. He was one of two Vietnam Veterans in my battalion who were still on active duty (this was the early 90’s). At that time, AR 670-1 only authorized WWII Marine Corps veterans to wear Marine Corps SSI-FWTS on an Army uniform. Everyone knew he was in Vietnam and no one ever corrected him, even during command inspections. It is up to the commander and senior NCO to enforce that rule in AR 670-1.

Capt Michael Wilford I was on a C.O.L.T in Ramadi in 06. Had a Marine K9 team attached to us all the time. I met General Peter Pace in 2007 wearing the 1st MEF patch. He shook my hand and gave me his coin. Wore it until 2010 when I got out.