The trouble is that Mum is there all the time and so the discipline etc falls on her shoulders. When Dad comes home from work and offers treats, be it extra tv, sweets, trips to the park etc etc she is seen as the bad guy and him as the good guy. Try swapping roles for a while.
I know someone whose little boy said to her 'I am not doing that because it is 'woman's work'! Obviously he had heard someone say that because he did not invent it himself.
When a Mum is exhausted from cleaning house and looking after little ones, making meals and doing the shopping it is all too easy to let Dad spoil the child when he comes home. Try off-loading some of the jobs such as shopping, get Dad to do it on-line, get a cleaner in once a week until the burden gets lighter.
Husbands think their job stops at the office when you have children, they need to be educated to the fact that when you have children it means you share the load.
As a Grandparent it would be better not to take sides. To offer help when asked, advice when asked, and be prepared to baby-sit now and again...or even collect children from school to give the Mum a break.
A woman cannot be expected to cope with it all on her own. When Dad comes home and undoes all the good work re: discipline, and a child knows instinctively how to play one against the other, then it leads to rows and hurt feelings.