Before Fiona attempted suicide, she was perceived by Marcus near the lake waving at him. He felt relieved.
Will also had gone through that in the supermarket, seeing his father having some products, and as he calls out to him, he kind of vanishes. I know it wasn't important. But did it mean anything?
I think the way they both had a similar experience was meant to show a connection between the characters. Maybe Will lost his father early in life and never got the guidance he needed. Whereas Marcus didn't lose his mum but she's not a stable, functional person so he's kind of on his own too (emotionally). The title of the film thus applies to both characters equally.
I think the scene also ties into the "need for backup" idea. I think there's more to it though, so I'm hoping others will reply here.
That's the most you'll ever get out of me Wordman. Ever. -Eddie Wilson
Not particularly, the whole film you get the point of view of the characters Will and Marcus. This is what they thought they saw, so you get to see it too.
I can't remember Will seeing his dad in the book. But the Marcus and mum bit was in there, but it wasn't particulary noticeable.