*Legally, no she can't do this - your partner can take her to court.
In practice, if she's determined to thwart contact, she can. My husband was in a very similar situation and has a court order relating to his kids from his first marriage. His ex has found ways of sabotaging contact anyway.
If he lets her dictate to him in this way, he'll never be free of her, and he won't be able to have a serious relationship until his kids are adults. So I'd argue that standing up to her is the right thing to do. But he may find she undermines or destroys contact anyway, court or not.*
100% this. Those saying its as simple as going to court or being adults have clearly never had to deal with a totally unreasonable Mother to their or their partners children.
In this world of equality and equal rights for all I am afraid to say that an unreasonable mother can still completely dictate what contact (if any) a Dad has with his kids. Even when the kids are saying themselves that they want to see him.
Thankfully most people are reasonable and behave in the best interests of the children. I would say there is actually something pretty unhinged about a mother who actively denies her kids a healthy relationship with a father who wants to take an active and meaningful part in his children upbringing.
Court orders, and parental responsibility count for nothing if the Mum wants to be awkward.
All the above obviously assumes the Dad isn't violent, abusive, constantly pissed etc etc.