Honestly, my answer would depend on two major things:
1] Why didn't B refuse? If they didn't know any better, or somehow C lied to them/manipulated them into believing only they could help, then I'd understand, and C would definitely be TA. But if they were aware of other options and still chose to put themselves through something uncomfortable for the sake of their friend, then that's...not really C's fault, technically? ^^; I understand why they'd feel guilty, but I wouldn't feel justified in calling them TA for it.
Like, if I take on the responsibility to do something for someone, I can't then turn around and blame them for asking me just because it sucked. That's not very fair.
2] How does B react??? I think that's pretty important...like, did they just suffer through it without having any idea that A could have helped in their stead? Or did they know beforehand? Or did they find out afterwards? And if they did, who told them? C? A??
Are they upset with C, despite the fact that they chose of their own volition to help? Or are they outright angry because they were tricked into it, like I suggested earlier? Or are they just ultra-forgiving and don't mind at all, leaving C to feel like they took advantage of their kindness?
Also, did A know about any of this? Could they have stepped in to spare B, and chose not to?? If so, they're as much TA as C is, possibly more...on the other hand, if they would have suffered just as much as B and didn't want to, I feel like that makes no one TA.
Also, also, what does this 'extremely uncomfortable' sitch entail? Are we talking "let me take you to an event where you pretend to be my s/o" or, like "let me demonically possess you and wear your skin into battle"?? I feel like that might change my answer a bit...
...In short, there are actually a lot of variables here. ^^; Depending on which ones actually fit this specific scenario, I might just go the neutral route, and tell C that they're allowed to feel guilty, maybe they actually are guilty to some degree, but it's probably something they can work through and get past if they're honest and talk with their friends. Y'know, try to give A a chance, if only to prevent this from happening again. ^^;