The OP did indicate that this is their best friend, not a simple business partner if you ask me. Though they did enter in to a business agreement when they opted to buy the week from Marriott. I don't think an formal written letter sent certified mail is justified here. These are best friends. Surely they can work something out.
Though I don't see an easy way out of this and remain best friends. It is possible, but a sticky situation. The only way I really see this working out is if the OPs friend is still in the rescission period. If so, have the friend rescind immediately. Then work out the ownership situation of the legacy week. Once that is squared away, then the friend can go back and buy the points if they still want them.
If outside of the rescission period, here is how things will likely shake down.
Situation 1 - The friends thinks the OP should just quit claim the entire week over to the friend and they are now both Premier owners in their respective years. This doesn't impact the OP much if they don't use points, but helps the friend some in those years the points are theirs. This is a viable solution, but the OP would need to have a written contract with their friend. Though if anyone breaches that, the only solution is to sue the other partner. No longer friends. The OP also loses official control and the friend can now sell the week and walk away with all the proceeds leaving the OP to sue. No longer friends.
Situation 2 - The OP decides that they don't want to deed their rights to the friend. The friend is mad because they sunk $40,000 for 3,500 trust points that don't have the Premier moniker. No longer friends.
Situation 3 - The OP decides to sell out their interest in the week. The friend is mad because the cost is too high and doesn't think that is necessary because they can go along with option 1 just fine and dandy. The OP doesn't like the drawbacks of option 1, so there is no transfer of ownership, no longer friends.
Situation 4 - They decide to sell the week to a third party. This isn't a good option since the enrolled legacy week is lost and they can't get another one to replace it for anything close to the going market price for a MKO MV week. The friend doesn't have Premier status. No longer friends.
I hope they are real good friends, but they say that blood is thicker than water. Even if this were a family relationship, it would result in bad blood (pun intended) in the end. The best way to solve this is to rescind if it is still an option. If not, then Situation 1 is perhaps the best option if they can come to an amicable and written agreement.
Though I would still be upset if my best friend did this to me. They are a 50/50 partner in the business agreement but the friend made a decision without 51% majority. Not cool.