If you guys are just cruising and not hauling ass, then the Diamond Back will probably be fine. If it winds up getting fast or competitive you'll probably be disappointed and lose interest.
Helmets, like seats, are all about fit. I'd go to a shop and try on several sizes and brands. For me, large Bell helmets fit best. You want it as tight as possible without being uncomfortable. It should sit even and level. Once you have it adjusted you should be able to shake it around a little without it moving much. This is so it doesn't get pushed out of place in a crash. After finding one that fits right adjust the straps so they hang between your neck and jaw and tighten it as much as you can without being uncomfortable. This prevents it from falling or ripping off in a crash.
Most better helmets now have a little piece in the back, often adjustable with a dial, that really helps to lock it in place so I'd recommend getting something with one of those.
Here's REI's advice for fitting a helmet...
http://www.rei.com/expertadvice/arti...le+helmet.html