I've always felt that Matheson telepathically picked up on Galen's memory, due to his strong emotional outburst.
I think he deliberately tried to help him overcome his long held grief, by bringing that "message" that was picked up by Excalibur.
Whether or not he made it up, found something pertinent, randomly by happy coincidence,
....or recieved a deliberate transmission by Isabel from beyond, is an entirely different matter and entirely up to the individual I guess.
Which strong emotional outburst?
When Galen was about to cast a fireball at the alien, and Gideon and Matheson stopped him, and Galen left?
When Galen crumpled up and discarded the message that Matheson handed him?
I don't think that Matheson would have made it up or found something pertinent, randomly by happy coincidence, and used it to help Galen overcome his long held grief. That would be manipulation of a friend and crewmember, regardless of the good intentions, and I don't think Matheson would have done that.
I think the alien sent it (as Galen suspects)
but it might be that the alien was a conduit/facilitator for Isabelle's message (like Lochley was a conduit/facilitator for Kosh's message to Sheridan in Day of the Dead.). Matheson just brought it to Galen, not knowing beforehand the context, but may have picked up on it after he gave the note to Galen.
Regardless, Matheson's empathy and understanding is sincere.
Is that actually mentioned by Galen or is it something you believe is implied by his reaction? I'd never considered it.
Whereas I think he knew exactly what he was doing with that note. There is something about the way he says "I know", which leads me to suspect it has two meanings.
I don't doubt that at all. It is clear that Matheson sees his ability as a gift to better the lives around him, when he bends the rules, he does it out of a genuione compassion and need to benefit their lives.