a 6" JL W3 is $1k....... if he needs to get two then the flat JL is the best thing for him..... jus have to put out a lil more
If you want the best bass possible you can't use the shallow subs...... they are acceptable for music in applications where limited space is available. If you want better bass sacrifice space. Putting two JL in the doors will require alot of work to the doors though..... I dont think he needs that.
On the other hand (sad to say

) just bought a 10" flat primere for dads van, will let you know how it turns out if you still looking at getting a sub (don't worry I'm not a pioneer fan but as far as stealth installs go this was the economical choice)
[$875 @ tripple R for a 10" flat] ----- same sub sells for $1150 at valpark sound and security I think unless is the 12" (which is $1049 at RRR).