Pacun, thanks for delving into this. I'll try to define in answer to you.
Partner C put down $25K from his pocket for 25% of the shares, correct?
There is no evidence that Partner C put down $25K from his pocket. He may have put in any amount, or may have put in nothing and this could have happened several years ago. The only facts are that his basis prior to default was $27,500 with loans attributable to $37,500.
The bank will not issue him a 1099-C because the loan was repaid to the bank.
Not stated that the loan was repaid in full, only that the partners were approached and C could not pay anything.
Technically he just lost 15% of his interest on the partnership. He only has 10% of interest and this is the way the new interests should be stated:
You could be right. Don't know that his default (with no other contributing circumstances) disqualifies him from being a partner or reduces his stated interest unless he is forced out by articles of partnership agreement. The question only involves his suddenly "overdrawn" basis and how it is handled on his personal return. Actually, there is also movement in the partners' capital accounts which is different from basis.
This is heavy stuff (especially for people like myself to understand) so I have a feeble mind when thinking about it. Thank you, Pacun.