Fire lots of it. In all seriousness though. SL or UD or SL/UD Split? The split would give you more options, bigger boost, and better faction buffs.
That being said what kind of subtye are you thinking of? Fire is so extensive you can make subtypes. Are you thinking straight up fire damage or do you like to play with lava?
Haven't had time myself to put a fast one together for him. I was going with an Acolyte, Terrain sub-themes.
When I play with lava I like to run eaters and just drop lava in the middle of them and the enemies. My guys get healing the enemy gets panicky being thrown knee deep in negative terrain, good fun all around. Since he mentions the balefire deck and he's a new account I assume he's a new player, I don't know if acolyte champs will be very accessible to him, but if you play any UD I'd think at least getting a sheoul demon would be a priority he's sexy. Edit: I see his forum account was made 3 months ago so maybe they're more accessible then I think.
Even still, with basic farming he has access to Flowing Embers, Demonlichs, Ash Maidens. My Fire BG does not focus on Amp so much as Utitlity, UD has plent of damage. Let me give you something similar to what I run. EDIT: Here is the budget version of something similar to what I would run. http://poxbase.com/deck/CjBC6-c6MCkuC32C8pch2C3Mc5=sys2ls8fs3oS86sOsAs70r3qrAr20e24e2ge2K I would recommend molding it to your own. I am a rather huge fan of Cleansing Flame So much utility in 1 ability.
You can earn ~3k in gold a day, just from dailies, and a few pvp/skirmish games. Between Extended Packs, and rune gifts froms Skirmish, sharding what you don't need, or is not trade-able. ~2 weeks. Less if your lucky. If your serious about farming, There is a guide on farming the Euan Campaign for some serious gold. Extremely repetitive, and boring though.
http://forums.poxnora.com/index.php?threads/thinking-about-fire-amp.9569/ http://forums.poxnora.com/index.php?threads/battlegroup-time.9523/ Both more "meta fire". Abyssal Fiends are too good to not include them in a fire bg.