I have a couple of opinions on this topic.
First of all, who hired him? And who hired you?
If it was the B/L, then she is also responsible for "firing". If that's what is collectively decided.
Many years ago I worked at a company where I was asked to deal with letting employees go that were not working out. After a time, I said to the company owner "If I don't hire them or have a hand in hiring them, I won't fire them". This actually helped as he realized he had to deal with the consequences of his human resource actions, and he had to vet the hires more carefully.
So, that's my first point, if you didn't hire him -as implied by the fact that he helped you get hired- why are you even concerned about the process of "firing"? Is the B/L afraid to deal with the issue? If, so, she is going to have to learn how to deal with that. You are entitled to have input into the band direction, but that action should be taken by the B/L, assuming she claims that role, and not just the easy part of the role.
Secondly, volume. If he plays too loudly, say so. There are different kinds of sticks, brushes and skin-knockers for all occasions. Try out a soft song with bongos or djembe and such. I'd bring up a song that would work with the trio, have her send him the chart, links to you-tube or whatever, and rehearse it. Best if it was not a 4/4 beat. From what you described him as, I'd call him a 4 on the floor drummer. 1 beat. 1 volume.
I auditioned for a band like that once. Guy miked his drums through the PA in a room above his garage. Then proceeded to play "Honkey Tonk Women" at 165 bpm as loudly as possible. When I pointed out the obvious faults to tempo and dynamics the response was "That's how we play it!" There wasn't one tune on the set list that had 3/4 or 6/8 time at all. Or one played below 150 bpm. I'm sure it would be no surprise to you that after I loaded out, I never went back. There was going to be no way to get this drummer from "Pounder" to "Tasteful".
After one or two attempts at the more tasteful pieces you want to do, if he can pull it off - problem gone. If not, then the B/L should sit down with him in person, and discuss the issues with him, allow him an opportunity to say whether or not this is a direction that he wants to pursue because this, as B/L, is where she wants to go. If not, than he has resigned gracefully. If he wants to pursue it, but the feeling is that he doesn't have the chops for it, then just be frank about it and say that the skill set required to handle the new tempos is not part of his tool bag, and that some of your shows will be as a trio going forward.
If the B/L wants to fire out and out, then just be frank and honest and say so. Be sure to leave a little dignity for him on his side of the table. It's no fun to be fired, but it is much worse to be fired poorly because people are too ill at ease to confront the truth head-on.
