No problem, as long as the cameras were properly white-balanced. Otherwise you are in for tedious CC work.
If he's wanting to mix different standards of footage (HD/SD) he'll need to do his final edit in an SD timeline or be prepared for some serious work in uprezzing his SD! But, as Harm says, as long as your standards are the same (all HD or all SD) and he's properly color balanced, you can pretty much always mix just about anything... I routinely mix JVC GY-HM700u footage (XDCAM-EX codec at 1080p) with Canon XHA1 footage (HDV codec at 1080i) and Sony handycam footage (AVCHD at 1080i) with little troubles...
It's pretty much common sense, really. Ideally you'll want to use identical cameras with identical settings. You can get away with certain differences, but those differences will add varying degrees of difficulty to your wok flow. Here's the priority you should be working towards.
1. Same model camera, with all settings identical.
2. Same resolution, frame rate and format, but different model cameras.
3. Same resolution and frame rate, but different formats.
4. Same resolution, but different frame rates.
5. Different everything.