@Cheeeeislifenow
Wow, I would have gone fucking mad.
OTT or not. How DARE he sign a card on your behalf and send it? Sorry but that is a majorly shitty controlling bastard thing to do.
My DH has form for this (Not often but occasionally!) Doing stuff because HE 'thinks is for the best...'
He also insists on sending Christmas cards to people we don't even like, we haven't spoke to for several years, and we unfriended on facebook in mid 2017, (and also people I have been trying to shake off for a year,) 'to show there is no animosity,' and 'to be the bigger person.'
He ghosts people, and badmouths them, and then wants to send them a Christmas card!!!
I argued about how stupid it was, and how it sends mixed messages and messes with people minds etc... But he said 'I don't CARE. We are sending one!' So this year (when he suggested it,) I said 'OK, let's be the bigger person, let's send cards to
Then I wrote them out, and went off out to post them, and threw all 5 cards in the bin down the road.
Don't know what advice to give OP, but I would have gone BATSHIT. How dare he do that? 
I think I would be very tempted to drop a note to said ex-friend and say 'sorry about your mother, but I did NOT send that card. A well meaning but interfering relative did it thinking they were doing me a favour. Look after yourself, and take care, but I won't be contacting you again or coming to the funeral.'
That's what I would do anyway.