The concept of "joint and several liability" means that either party can be held liable for the whole of the debt either jointly with the other party or on their own. On any debt, this effectively means that the party that is traced can be made to pay the whole debt, whilst the party that is NOT traced can escape by paying nothing - this does happen!!(posted 7173 days ago)In your stated case, they have traced the ex, she has made payments and thus, entirely on a voluntary basis on the part of the BS, her liability has been limited and now they will no longer chase her.
There is plenty of stuff on this site regarding the limitation period which, if this debt is 13 years old, your boyfriend might not have anything to pay at all!! (even if you have offered to pay outside the 12 years, it is still statute barred as the debt is still there - you just can't be forced to pay it).
Get some advice on this matter from either the CAB or a good solicitor who could negotiate a settlement through or advise if it is indeed, as I suspect, statute barred. Also, if it is not SB, then a challenge as to proof of the debt sometimes causes a reduction in the amount asked for.
As to using the letters in court - well you can, but I do not think it will do much good except to show what might amount to unfair treatment of the second debtor (your boyfriend) as against the first - but it does not alter the legal position of, in this case, several liability.
Get some advice pronto!