The Christian life is not based on avoiding the truth but on hearing and submitting to it. The greatest kindness we can offer each other is the truth. “Then you will know the truth, and the truth will set you free” (John 8:32).
I would come humbly, being sure my own sin is confessed and ask God to do a work of grace in the person I’m talking to about the grumbling. Then, if they don’t respond well, continue to pray and give God time to do the work that only He can do. Sometimes people surprise us and accept the truth, other times they reject it at first but accept it later. In any case, we need to be faithful to speak the truth in love.
For more information on this subject, see:
A 12-Point Cure for Complaining by Bill Izard
Biblical Principles for Godly Verbal Communication by Randy Alcorn
Does It Matter What Others Think? by John Piper
Longing for Joy by Randy Alcorn