Sorry, this is not a "special" guitar, and I do collect Martins.
Other folks like Wade have already covered the 70's part, and as mentioned the enlarged soundhole is a detriment not a plus.
If done at the factory, and I agree Martin probably would not have done it in those days, the guitar would be stamped "custom". A modification by the best luthier in the world is still a negative versus having a pristine stock instrument.
Appraisals (and not just guitars) are typically very high and do not reflect the real market, the appraiser gets paid and the owner is happy, but insurance claims will only reflect a percentage of the inflated appriasal.
That guitar is worth less than a typical 73 Martin with the enlarged soundhole, and early 70's Martins are not held in high esteem and prices are lower in general than other decades.
If your friend thinks he has a special guitar he will not be happy with what it is really worth, I would back out of selling it for him, it's a losing proposition.
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