if he goes to a party like THAT, he’s gonna be in over his head

[meliss]

Hi, the narrator, Rowly and Mariana are middle schoolers. What could mean "if he goes to a party like THAT, he’s gonna be in over his head"? He is referring to Mariana’s Halloween party.
This year, Mariana’s parents said she has to keep it SMALL, so she’s only inviting people who are in the school band with her. That’s bad news for kids like me who were hoping to get invited this time around. Rowley’s gonna get invited, though, because he’s in the band. But believe me, if he goes to a party like THAT, he’s gonna be in over his head. (Diary of a Wimpy Kid, Double Down, Jeff Kinney).

 

[J&K Tutoring]

We need more background to understand. It can mean:

1. If he goes to that kind of party...

2. If he goes to the party in that way (describing him somehow- probably the way he is dressed)

My vague recollection of the character Rowley makes me think it's #2.

 

[meliss]

2. If he goes to the party in that way (describing him somehow- probably the way he is dressed)

My vague recollection of the character Rowley makes me think it's #2.

There were no references to anything like that so far.

 

[jutfrank]

I assumed number 1 (from post #2), which I think is far more likely, given only the context that we have here.

 

[Tarheel]

There are no references to anything like that so far.

Context should make it clear which meaning is intended. If it's not about the way the person is dressed then it's probably about what kind of party it is.

 

[J&K Tutoring]

This discussion reminded me that I have a copy of Diary of a Wimpy Kid, so I dug it out. I am very confident that my #2 is correct.

In the stories, Rowley is the not-very-bright friend of Gregory, a middle-schooler who wants to be a 'cool guy', and more grown up, but Rowley seems perpetually stuck in about the third grade.

There are several books in this series, and I don't have the one with the passage cited in the OP, but my book does have some Halloween stories in it. Gregory is at the age where he wants to stop wearing a silly costume but still somehow get the candy or go to the local Haunted House. Rowley shows up in last year's costume- Superman.

 

[jutfrank]

I didn't realise you had such a high-brow taste in literature, J&K.

I think that proves that your interpretation was right, then.