You are right in that he shouldn't just drop his friend because his partner is starting to worry.
He should though reflect on why his relationships are being lost because of his relationship with his friend, whether his friend was a man or a woman. If it's just his previous GFs are insecure then he needs to think about why he is attracting or attracted to insecure partners. If it's because he drops plans for a friend who only needs to crook their finger then he needs to understand that's not a way to have a healthy relationship with a partner.
There is nothing to say that he is or wants to have a sexual relationship with his friend but he is in a heavily emotional relationship with her if he is dropping plans for her. Emotional affairs can be way worse to cope with than sexual ones because they are not easy to detach from.
I've had a relationship where a friend (male) was XBFs primary relationship, while it was fine most of the time, I did get fed up of making plans with BF only for that to be scuppered by friend who didn't like me so the moment he thought BF was getting too close he'd draw him away breaking plans purposely to make me break it off. While I don't know if this dynamic happens in OPs DP relationship with his friend or not, it is a possibility that the friend is not overly happy with having her support and cheerleader taken away from her and may "need help" at times when DP is getting too close to his GF, she may not and be perfectly innocent too.
We don't know enough from OP for a judgement call on it really there are so many variables but no he shouldn't just drop his friend but he needs to know why he is so willing to run to her when he has other plans and why his partners find that unacceptable.
Also if she is an important person in his life, why are they being kept so far apart. I would expect someone you are thinking about committing too would meet your best friend quite early on whatever sex they are. That way there is none of this wondering what she's really like or what their relationship really is.
At best the OPs BF has very lax boundaries with this friend, at worst she is his emotional partner.