She is helping her parents from her salary without her husband’s knowledge

If there was no condition
stipulated by your husband that you may work provided you spend all or part
of your salary on household expenses, then the salary that you get or the
amount that is left after you pay the amount that your husband stipulated
you should pay, is your property and it is permissible for you to spend it
in any way that is halaal, and you do not have to ask your husband’s
permission with regard to that. But if you do that so as show respect to
him, that is better. 

Helping one’s parents is
one of the greatest acts of kindness and goodness, and it comes under the
heading of upholding ties of kinship that increases one’s lifespan and
brings blessing in one’s provision, as the Prophet (peace and blessings
of Allaah be upon him) said: “Whoever would like his rizq (provision) to be
increased and his life to be extended, should uphold the ties of kinship.”
Narrated by al-Bukhaari (5986) and Muslim (2557).  

Based on that, there is
nothing wrong with what you are doing by helping your parents with part of
your salary without your husband’s knowledge, especially since you say that
he does not mind, but if you think that telling him will make him feel
respected, then that is better. 

We ask Allaah to reward you
for this help and to bless your wealth and your work for you. 

And Allaah knows best.