Not that familiar with Marine Corps protocols for handling wounded and unit records. In the Army there is a Morning Report which lists any change of personnel status in a company size unit each day. So if someone is wounded and removed from the unit for treatment it would be recorded the next day. If the Marines have something similar I would go to where those records are kept and get copies of the time period.
NARA at College Park has the following but I am not sure if it holds the Morning Report type records.
127.3.2 Personnel records
Textual Records: Records relating to officers, consisting of rosters, 1821-50; registers, 1819-48; military histories, 1869- 73, 1899-1911; and monthly reports, 1821-1911, with gaps. Records relating to enlisted men, consisting of service records, 1798- 1906 (490 ft.); size rolls, 1798-1901; descriptive lists, 1879- 1906; and an alphabetical card list, 1798-1941 (367 ft.). Registers of courts-martial, 1897-1906, 1919-33; desertions, 1809-1907, 1910-41; discharges, 1829-1927; and deaths, 1838-1942. Casualty card lists, 1776-1945. Strength and casualty reports, 1775-1971. Muster rolls, 1798-1945 (1,285 vols.) and 1798-1953 (4,172 rolls of microfilm; see note under Related Records below). General returns, 1821-1914. Certificate books containing service information, 1837-1911, with gaps.
Microfilm Publication: T1118.
Oh and all Marine Medical Treatments were done by the Navy so you have to find out which Navy unit was supporting your father's Marine Unit to track down those treatment records
LTC USA (Retired)