What is the meaning and implication of 'ローンボーイ'?
We can all agree it's a pun-ification against the "lonely guy".
There, I just made up a word "pun-ification" which won't appear in search results, but it's reasonable enough to convey the meaning. That's exactly what Mion did. She stitched together two words to make fun of either:
- Lone (as in lonely) + boy
- Loan (as in borrowing) + boy
This is because lone and loan (or even lawn) can all be written as ローン just because that's how katakana works.
In the end, Rena is clearly treating it as "loan". In that case, and in this context, "loan boy" would mean "a boy who is broke and needs to take a loan". Remember, it's a made up word after all.
If you've played the series, you might've noticed how smart Rena is. Their IQ has been published officially and sure enough she is clever, while Mion is average. Considering such character settings and how Higurashi is written, it may be likely that this scene describes the following:
Mion simply changed "guy" to "boy", in order to say something like:
"Lonely guy? More like lonely boy"!
And then Rena added an extra twist:
"Loan-ly? Are you short on cash?"