One time, another leader suggested to me taking a folder and putting "Complaint Forms" on the outside and filling it with blank Leader Apps...
I'm going to take a different angle from a lot of people on this one.
If they're help is welcome, then you already know what to do-get them to volunteer.
If they do volunteer, will their changes be welcome, or is the Pack Leadership happy with how things are progressing? Most of us will say that they need to jump in and help the Pack, but if the rest of the Pack is happy with everything and they are not, they will get frustrated and you've solved nothing.
If not, then another suggestion would be to move to another Pack. We have had a few parents and Scouts who were on the verge of quitting altogether; we suggested they try out other Packs and find one they felt was more in agreement with their personal "style". They did and began to enjoy Scouts more and the parents ultimately became co-Den Leaders.