Last words are normally directed at people you love like your significant other, parents, even a pet, but rarely at a random friend.
Except she's not a "random friend". She's his roommate, has been for a long time and his best friend. With Angus and Faye's relationship still being young, I'd almost say they're the most important people in each other's everyday lives and yes, it's possible to have that bond without romance.
I would consider random someone like Natasha who we haven't seen in a long time or someone with very little interaction with Marten lately. I repeat that Marten and Faye live together so even though we haven't seen much interaction in comic, it's always implied they are just because of their living situation.
I'm not sure what moment of weaknesses there have been other than when Marten got drunk. There's been a lot more instances of Marten and Faye saying they're ust friends. We have seen
Marten talking to himself, when he thought no one could hear him so he has no reason to lie, confirm to himself he doesn't have romantic feelings for Faye any more. Possibly between the break-up and before Faye and Angus consummated their relationship, he didn't know how he would feel about her romantically. However he said himself he didn't think he would be bitter or jealous and quickly realized he really does just love her as a friend and is happy for her new relationship.
Also as others have said,
you don't throw a celebration for the person you're pining for having sex with another person and then do "the thing" with the other person.I would guess if your hypothetical scenario happened, Marten would not be happy. After all the time and drama it would not be a wanted event, especially when they have developed the different, non-romantic bond that they have now.
*EDIT: Whoa, sorry for the textbrick.
I agree with Skaltura. Maximum ridiculousity is needed.