"Hiiiiiiiiiii Griiiiiiiiiiim, I maded this kewl rock out of twigs for Mandy, and this fungus hamburger for Mandy, and this squirrel underwear for Mandy, and tell Mandy that I miss her and that I'm busy making her a matching squirrel hat, and.."

Billy kept on stuffing Grim's arms with all sorts of awkward objects, not noticing Grim's entire body was already pulverized by the weight of the "gifts". Still Grim dared to ask him why he didn't make him something.

"Oh, sorry Grim. But I still get to see you this vacation, so I thought.."

"It's fine, boy, I understand Death doesn't beat a pretty face."

"What? No, it isn't like that!"

Grim decided to change the course of the conversation before Billy would go on his "MANDY IS NOT MY GIRLFRIEND"-rampage and started beating up everyone in sight. Again.

"So, you've been quite busy, and dis is just your first day of camp."

"Yeah, it's really fun here, we can do anything we want. I wish Mandy could be here."

"Well, dis is a boys camp, after all."

"What does that have to do with anything?"

"Concerning Mandy, not much."

Billy obviously enjoyed Grim's company, and tried to stretch time by giving him a tour of the camp, which turned into a tour of the entire forest, which turned into a tour of the whole town. Grim knew he was actually very lonely while he claimed not to be, but still didn't want to stay any longer since he didn't really had the need to meet with every rock they passed. And there was a comfy couch and a television set waiting for him at home.
After a short while he was getting noticably fed up by his neverending tours and rants, but when Billy saw he was about to say goodbye, he quickly asked:
"So, what's Mandy up to?"

This is one of those questions people ask with no thought.
This is one of those questions people ask you frequently on MSN, and that way pretend to be interested in you. But they don't care about "what you're up to", it's a mere conversation-starter. But was he asking it just to keep him here, or was he really wondering?
Because in this case the question had a serious answer to go with, but Grim wasn't sure if it really concerned him in any way. Even so, it wasn't like it would hurt him; if Billy's day would turn a bit brighter by these extra seconds of jibber-jabber, then Grim was willing to rat Mandy out.
...
Yet it stayed quiet. He didn't say anything, but was busy thinking to himself how to say it.
Then after 10 long seconds Billy broke the silence:
"She's up to no good, huh.."

This surprised Grim.
Billy was far from intelligent. The only reason he hangs out with the cold-hearted Mandy is because he simply doesn't see that she is cold-hearted. He describes her as "grumpy", but not "evil".
How does he even know Mandy is able to be up to no good? He hardly knows what "good" and "bad" is. Next to that, he never payed attention to her sick obsession to take over the world, while it stared him in the face multiple times.

"Well, yeah." Grim began, "Yesterday the girl didn't leave her room once."

"Maybe she misses me, too?"

Ah. Figures. He felt rather stupid now, to think that Billy was actually able to observe.
His mind was somehow never able to portray Mandy in any negative way. His look on "being up to no good" probably doesn't go further than putting a bucket of glue on someone's bedroom door.

"I don't tink so, Billy."

"Can you ask her when you's back?"

"Fine, okey, yes, can I go now den?" Grim said annoyed.

They said goodbye and Grim teleported away. When looking around, Billy noticed he was standing in a supermarket in New Jersey.
"..A kickass tour that was!" Billy said cheerfully, and went to the nearest busstop to wait for a bus to bring him back to the camping site.