To be fuming at my friend

It's matters little what the health level of the excuse was, it's the short notice cancellation that's rude. But there's little you can do other than never book anything on behalf of that person again. I'd hope you could bring a last minute replacement or just enjoy it alone or give away both tickets? It's not good but if someone's claiming illness then you have to just accept it.

Take someone else next time. They should also be offering to pay for their ticket, at least.

Wasting your leave is really irritating, that would annoy me even more than the cost.

I'd be pissed off too. They should offer to reimburse you the money you paid for them to go as well.

Seriously?
What about OP’s ‘mental health’ or doesn’t that matter.

Mental health issues are as important as physical health issues.
I would be concerned rather than cross.

I can see you must have been incredibly disappointed and 'fuming' at the situation. But whether you should be angry with her depends on whether she has a history of debilitating mental health (in which case you probably should have half expected it) and whether you actually don't really believe her, or think its an exaggeration or excuse. You know her. If her illness is genuine then sympathy is in order (I know it was rubbish for you, but imagine finding you literally can't face going to something?) If you think its not genuine then you probably need to review your friendship.

I can understand you being disappointed but not 'fuming' unless you think she's not telling the truth?

I wouldn't book anything again with them in the future though, just stick to coffee or cafe lunches etc.

Depends how severe their mental health issues are.

Speaking as someone who has a mental illness:

If their mental health issue is that they're feeling a bit anxious or worried or miserable, then yes, I'd be annoyed.

If their mental health issue is that they're having a psychotic episode or a full-on vomit-inducing panic attack or they're hearing voices or are suicidally depressed or have cripplingly bad OCD, then I wouldn't be annoyed.

Well no, with vomiting you'd cancel when it became apparent you couldn't go. That might be 30 minutes before you need to leave if that's when you vomit for the first time.

Does your friend have a long standing mental health problem? Or is this something totally out of the blue?

I really sympathise with you when you have put so much effort into arranging this outing and it's been planned for a while. But if you know that this a genuine reason for your friend cancelling at last minute I think you will need to accept that she isn't doing this purposely to upset you, that it is something she can't help or control. And she probably feels awful about letting you down.
Fwiw at times, especially when I was younger, my absolute terror of social situations would mean that sometimes I couldn't actually go to events I'd said I'd go to/ bought tickets for etc. Part of me really really wanted to go but the fear trumped everything.

I'm not sure i would be "fuming" but I would definitely be annoyed

It's unlikely that MH problems only presented today so very thoughtless to cancel so close to the time of the event when its likely too late for you to find a replacement guest to take.

Having said that, If she's usually a reliable and thoughtful friend then I would give her the benefit of doubt on this occasion.

I’d feel cross as well.Maybe if they’d cancelled earlier, you’d have time to find an alternative friend to take. Did they cancel first thing ( which you would do with vomiting etc), or later in the day?

Replace mental health with puking my guts up / have broken my leg - how'd you feel then? I'd be disappointed (and concerned) but probably not angry. Although if it was a friend who cancelled all the time for no good reason (ie not that they are genuinely struggling with their mental or physical health) then I'd be miffed. Hope you enjoyed the event anyway.