Who exactly was the boy Petunia mentioned?

[Shelia]

No: HPFGUIDX 73887

I read this on another site, so the general gist of the idea is not 
mine (gives full credit to thinking mind of others) However it was 
not discussed, just thrown out there and I would like to see some 
theory behind it (if any exists, and it may have been touched on here 
and I have not seen it as the posts are overwhelming)

When Petunia mentions Azkaban, the others want to know how she knows 
of the place.  She said something about 'that boy' had told Lily 
about it or something to that nature..

Was 'that boy' really James?

When did this discussion take place? We now know that Lily and James 
did not become an item until their seventh year.  If it was before 
that then perhaps it wasn't James.  

Could the boy be perhaps another?  Lupin? Or maybe Snape?  Did Lily 
have another 'boy' visiting her? (I know Snape would not see a 
mudblood or would he?)