A friend of mine has been setting up his studio for the past several months, and the only thing he still needs is an interface. He has a Mac, so I really had no recommendations for him.

He wants somewhere between 16 and 24 inputs, because he's planning on tracking bands live. He doesn't care what software he ends up running - whatever works.

Any recommendations or things for him to keep in mind?

Lots of options... There are probably cheaper as well as more expensive setups I could list
but since you didn't specify a budget I just went with a general cross-section.

MOTU 24I/O Core PCI-e (w/breakout box) $1400
*A/D conversion only. No pre-amps. 24 simultaneous inputs. Mac version includes "Audiodesk." Compatible with any ASIO DAW.
Total Cost $1400 + Whatever Pre-Amps you get.

Mackie "Renegade" Rig: I'd totally use this if all I recorded was hardcore and punk bands.
Mackie 1200F (Comes with Mackie Traktion) $1600
*12 built in onyx "boutique" pres, expandable to 30 ins and 34 outs.*
Total Cost: $1600

RME Budget Box: Extreme Low Latency, Decent price
RME Fireface 800 $1499 (8 ins/outs)
ADI-8 $1799
*These two together supposedly give you 26 simultaneous inputs.*
Total Cost: $3200 (cost is before ASIO compliant software... Cubase, Sonar, Logic, Nuendo..etc.)

Digidesign Budget Box: The "Vanilla" of the recording world
Digidesign 002 rack (8 inputs out-of-box but only 4 pres) $1200
Focusrite Octopre (w/ optical outs)
Apogee mini-me $700 (EBay) (w/ spdif coaxial)
Total Cost: ~$2600

Logic Pro Box: I can't use logic since Apple bought them. I keep thinking I should be wearing a black turtleneck and wearing a funny hat.
Apogee Ensemble: 8 high quality pre's out-of-box for $1,949.00
Add a second box for $1,949 for 16 ch.
Logic Studio 8 $499
Total cost $9000

Pro Tools HD Box: Everybody knows it. Everybody wants it. If they say otherwise they're lying. Or French.
192 i/o (w/ 8 Analog ins + 8 Dig) $3900 (retail)
Focusrite 8 channel pre w/optical out $700
HD1 Core Card: $3000 (Ebay)
Total Cost: $8000