"loose"?? Is it still attached? Does she have the key..(the tuning wrench that fits over the pegs for tuning?) My wife uses metal picks, but it can simply be a matter of personal preference about the sound.

Getting the 'right' replacement strings for and old zither can be tricky... and it is often useful to have someone with experience, if at all possible, help you with the details of changing and adjusting strings the first time.