I personally DPS when I'm healing because standing around gets boring, but I don't really think you should force a healer to DPS if they're not comfortable with it. Particularly in dungeons they're new to, since they might lose focus and get tunnel vision or panic and forget they're in cleric stance when healing.

At the bare minimum, the tank's job is to grab mobs and take damage, the DPS's job is to kill stuff, the healer's job is to keep everyone alive.