That sucks. Does it sound like the recipient is super invested in canon, or are they into it for nostalgia reasons? Because I know that I would happily read fic for some of my old shows, even though I can barely remember what the character's names are for some of them. If you can just write from that sense of nostalgia, without worrying about how bad the actual show is, I bet you could still write a nice story, one that you'd both enjoy.

Otherwise, I'd say don't worry about it. A treat is a nice thing for you to do, but your intended recipient isn't going to know you didn't write it for them. You're not an awful person for not writing it. :)