Sorry to hear of the loss of your friend. As you can image its very difficult to provide an accurate estimate without several
photos of the rifle. Condition dependent the rifle could range from a few hundred to a few thousand. Even higher if the rifle possess special features, such as engraving, high quality or checkered wood, takedown mechanism, half round half octagon barrel, gold or platinum inlays....etc..
Options
1) Post photos (or get photos to someone here who can post them on your behalf) and folks can generally chime in....
and do with that what you want....
1A) out of that posting see if someone here is interested.
Generally someone should tell you want its worth and what they would be willing to pay (understandable if those values differ slightly)
just like selling any item...its a negotiation.
2) If you have some internet savy there are a few reputable online sales sites, such as Gunbroker.com or Gunauction.com
You need to be able to set up an account and provide a description and photos. Upon sale of the rifle, you would still need to take it to a
local gun shop and use there FFL to transfer to the purchaser upon them sending the LGS a copy of there LGS license. These sites typically charge you a few percent based upon the total amount sales, shipping and transfer fees can be paid either by the seller or buyer, depending upon how you want to set it up. Just be clear on it. (typically max 5% or so lost to site and potential shipping cost ($60) and transfer fee ($35-40) on your end out of the overall price. You can sell it with a set price or set a reserve price... or sell it no reserve....make the sale last at least 7 days or longer to so that is visible for a while.
3) Pack around to a local gun show, and explain your situation. Please Please....Be educated on what is value is before your go this route and have a decision already made as to what you would take for it. Realize those folks need to resell it to make money just like a pawn shop or other business and there offer is a reflection of that.....often 20%-25% less is a realistic value from someplace like this... don't be surprised to get offered half or less than half of whats its worth)....
Keep in mind....there are a few of the folks behind those tables would sell there mothers kidney if they could make a buck...
4) Take it to a local gun store and see if they will consign it. (consignment fees can vary.... typically i see around 15%) the positive is the foot traffic....the negative is most likely lots of folks will handle it which can lead to a few more handling marks etc.
(not a good approach with a unfired firearm)
5) Auction houses...some will sell a single rifle if the rifle is nice enough...a lot is lost in this effort
Typically the seller loses (15-20%) to the house for the sale. and the rifle will bring a bit less than max. due to the buyer also having to pay a 15%-20% premium on top of the hammer. Unless the house is located nearby both will pay shipping costs....
Hope this helps.