Yep... so long as the card they're playing doesn't inherently imply that it should be played by the person making the check or otherwise say that it can only be played on a person at the same location (like cure's text for example, or Valeros' power of adding 1d4 to combat checks). Blessings, for example, can be played to help other characters at other locations.
Yes... usually. The only time you are limited is when the card specifically says "your location" on it.