For any significant gun - like a Purdy or a Parker - he should get a written appraisal on it - if he really wants to insure it. There are a number of specialty dealers around the country - or big gun dealers - that he should probably discuss it with at least before he pays for an appraisal.
He has 2 choices on his insurance once he establishes a written value for the gun. If he's worried about theft - then he needs to "schedule" it on his insurance policy - there won't be any deductible - and the value won't be disputed - if the insurance company accepts the appraisal and agrees to schedule it. If he doesn't "Schedule" it on his policy - then most policies have a sub-limit for theft of firearms ( for all guns - like $ 2,500 ) and he needs to discuss with his agent - after he has a written value on the gun.
If he's not worried about theft -- because he'll keep it locked up and probably not shoot it - then its covered like his furniture in a fire ...or for any peril other than Theft. He still needs to keep photos and the appraisal so he can establish value on a relatively rare item ...becaus insurance adjustors all know what typical couches, refrigerators or most standard guns are worth - but not something like this.
He can't inflate the value - he needs to rely on the appraisal / and keep it in a safety deposit box - or maybe even send his insurance agent a copy of it and discuss it with them. But while there is a sub-limit for Theft of guns on an insurance policy / guns lost in a fire are insured at full value...not necessarily replacement value - but actual cash value at least. But if its locked up in a good safe and just part of his collection - he never shoots it - and he doesn't want to replace it - he just needs to establish the value thru an appraisal and make sure the perpsonal property on his policy is high enought to cover his guns, furniture, clothes, etc.
In terms of value on it ...really hard to tell .../ but I don't think the value is going to be that high ...its a crossover stock on a 10ga SXS ....Parker or not ...there isn't much of a market for it. But maybe I feel that way - because it isn't something I want ....but he needs to first talk to a knowledgeable appraiser that has more experience with these crossover stocks than I do. In its current conditon ...it might only be worth a few hundred bucks as a novelty ...I just don't know. He can't insure it for sentimental value ...only its appraised value ...but if its only a few hundred bucks ...I wouldn't recommend he "schedule it" on his policy ....just treasure it / keep it as a keepsake from his family and appreciate it for what it is.