While I do agree that Faye would want to protect Marten, I think that's not the priority in this situation. Marten seems to actually be surviving okay after his initial depression, and as has been debated in many a WCT, is acting more assertive and confident than he ever has in the past. Dora, on the other hand... she's in therapy, she seems to avoid the topic, and she's still prickly as a porcupine on anything her brother does. Faye may suspect - and I certainly do - that Dora is currently far more fragile than Marten. While the Manatee on Codeine seems to have an iron core under that soft shell, it's looking like Spookypants is exactly the oppoiste.
That's not to say that I believe Tai would be bad for Dora, per se. I just think that anyone making a move on Dora right now wouldn't be good for her. And if Faye were to object to the pairing, I don't think the reason would be to protect Marten. She might say as much to avoid awkwardness, but I really don't think he's the one who needs protecting right now.