I can say that it's absolutely possible to have hearing loss due to a head trauma (including many types of car accident). I actually lost an additional 5db on my left side in a rear end collision almost 7years ago. Physiologically, it's probably more likely to have hearing loss from a side impact, than a front/rear impact, but any impact has the potential to cause temporary or permanent hearing loss.
Many people don't realize that "head trauma" (any type of impact/fall/infection causing swelling to the brain etc) is one of the leading causes of sudden onset (non-genetic) hearing loss in adults - we actually have a number of people here on this board who have loss some/all of their hearing do to falls/accidents.
Some injuries that have been decisively linked to sudden onset hearing loss are: concussion, whiplash, neck injury, physical impact to the head, hitting the steering-wheel/door/windshield, falling/passing out and hitting one's head on something 'solid', being hit by a baseball bat/ golf club etc.
It's also possible for an injury (or illness/infection) to cause what is called "fluctuating hearing loss" (which I happen to have). With fluctuating hearing loss, various things such as stress, health, environmental factors, sinus pressure/inflammation, foods &medications can cause a very real, measurable change in one's hearing. The changes might be sudden (over hours) or gradually change over days or weeks (making it more difficult to notice and diagnose).
In my situation, I'm completely deaf on my right side (more than +120db) and have mild/mod fluctuating hearing loss on my left side. If I had a hearing test done everyday for a month, the results for my left side would vary from about 5db-20db over that month. I'm not "faking" these results, it's a true physiological change in my hearing based on a number of factors (one of which is that I also have very severe Fibromyalgia with Chronic Fatigue, which has been shown to cause F.H.L.)
As far as "faking" an Audiogram ....
With a qualified audiologist it's VERY hard to "fake" an audiogram. There are a few reasons for this, including that there is a test call a tympanogram that measures how the ear drum responses to sounds - the person being tested has ZERO control over the results. If the tympanography results doesn't match the pure tone testing the Audiologist will do additional tests to make sure the results are as accurate as possible. Once all the testing is done, the Audiologist will put an "accuracy score" on the audiogram ranging from "poor" to "excellent" which indicated how confident the Audiologist is about the results.
For what it's worth, because of the type of hearing loss I have, the type of tinnitus I have, and because I have APD, the best results my Audiologist get for me are "average/fair" which means that it's about 75% accurate. This doesn't mean I'm "faking" it, rather it simply means that issues beyond the audiologist and my control influence the accuracy of my test results.
BTW
If you're trying to "prove" they didn't sustain any hearing loss (temporary, permanent, fluctuating, or functional) in the accident it's likely to be a long, expensive, uphill battle. You'll want to have your insurance/lawyer request they see an Audiologist specializes in trauma related hearing loss as well as auditory processing problems.
To be completely honest though, unless they have a job in which they are required to have annual or bi-annual hearing tests done professionally by an Audiologist in a proper office you're not really going to be able to 'prove' they didn't lose hearing in the accident.
Another thing to keep in mind is that even if the person had a "perfect" hearing test (no change pre & post accident) they may still have hearing/understanding issues due to the accident.
In some rare cases people who have head trauma (even a mild concussion, but certainly with any sort of bleed/fracture etc) may actually acquire CAPD/APD due to slight/mild trauma/damage in the brain itself (or hearing nerves). In these situations someone will functionally have a hearing loss (or altered ability to hear/understand - especially in noise) but not necessarily have db loss on a pure tone hearing test. It's important for people to realize that while CAPD/APD isn't a hearing "loss" per say, it IS a (sometimes major) communication loss/impediment because it alters one's ability to understand what they hear - effectively making them "functionally hard of hearing"
I hope that helps, although I'm guessing it's not really what you wanted to hear.