passport
school enrolment
bank accounts
joining some clubs (a neighbour of ours recently had to produce a birth certificate for under 6 soccer!)

If she is the sneaky type maybe trying to change their name or similar - but she needs your DH's permission for that (she mightn't realise though)

But all she has to do is apply to births deaths and marriages and she can get a new copy anyway. Maybe she doesn't realise this - tell her to do that instead of giving her your DH's copy - it may be useful to have in the future.