Well, that is a challenge, no doubt. I hope your friend is a good sport.
The Kingdoms of Men book will explain all about warbands. Of course, this is assuming your friend is using the newer warband system. If he's using those old rules too, then it's a bit simpler: carve up your army into quarters, lead each with one major hero and pack in as many warriors as you can. The old system is very "number hungry", and if he's fielding a Mumak or two then just think of all the warriors you can field instead…! Make sure you have 1/3 bows…if the old rule are in use, then volley fire is too, and that can be useful in the beginning of the game.
Otherwise, if he's using warbands, and this is your first crack at army building, I'd skip Citadel Guard and Osgiliath vets, and I'd skip Fountain Guard unless you're willing to put shields on them (the blisters don't come with them, but they can take shields as an upgrade). A mix of WoMT with shields and spears, Rangers of Gondor with spears, and a few Fountain Guard with shield will give you all the flexibility you need for your Gondor contingent. Some knights might also be handy, unless you want to leave the cavalry to Dol Amroth. Note that per the Kingdoms of Men rules, Boromir (Fellowship version) can't lead Gondor units, so if you want Boromir it's supposed to be the White Tower version. Since 1/4 of your army (625 points) is Gondor, Boromir Captain is probably a bit expensive. Faramir might be a better bet, along with other cheap heroes like Beregond, Damrod, and Madril or Cirion. In total you'll want something like:
1/2 WoMT (half with shields, half with spears and shields)
1/3 Rangers of Gondor with spears
1/6 Fountain Guard with shields
A natural complement to this would be Fiefdoms led by Imrahil. The other fiefdoms heroes (except the ranger) are good as well. If you take Imrahil you really want to maximize his banner effect, so using non-Dol Amroth warriors (clansmen etc) is kind of a waste (IMHO, not everyone agrees
). Because of the banner effect you can take a bit more cavalry than with normal heroes. All these guys have higher than average fight, and if they're within 12" of Imrahil they'll get a reroll on every fight. In contrast, the fiefdoms rangers are really nerfed, they can't take spears and have terrible courage, so I'd skip them entirely. In total you'll want something like:
Up to 1/4 mounted Knights of DA
1/2 Men at arms of DA with pikes
1/4 Foot knights of DA
Maybe that will get you started...