If it iwas cancelled with less than 24 hours notice (as opposed to your friend only finding out about it then) then yes, it is for BA to resolve. More than 24 hours it's for the OTA to resolve. I sincerely hope they haven't cancelled for refund the original ticket since then they will be unable to get reimbursement for second ticket (see Disruption Signposts thread). But having done what they did, it will now fall to the OTA to pay the new ticket, and I'll be surprised if they actually do so. The usual process is that the OTA would buy the replacement ticket and resolve with BA internally, so something appears to have gone wrong there. BA will potentially be liable for cancellation compensation of £520, depending on the details but I can't see them refunding a replacement ticket unless a BA agent - in words of one syllable - invited them to do this, and in my experience they would never do so. More usually your friend may have suggested this and the BA agent didn't say "don't do this".