There are a few points that are lacking in the OP’s comments. Is the band leader either of these couple? If not, what do they have to say about it?
Have you mentioned that her time might be better served getting her keys playing to a higher standard? Sounds harsh, but that would be my response.
Has she said why? This would be my first question. There is no reason to go down this path if you are a functioning band that’s getting gigs. It just doesn’t make any sense other to serve this persons ego.
I’ve been in bands which have couples in them and they have been a complete joy. I’m still in one and they are a fantastic pair. I accept other people’s mileage may vary here, but couples in bands are not always doom and gloom. Sometimes it can be a good thing.
Personally I think you’re making a rod for your own back by giving advice re gear etc. This will only fan the flames of someone who clearly must believe they can do a job as equal if not better than you. If she thinks she can have a go on bass and not be up to your standard, that’s kind of worse.
Sorry, but I have little time for these wannabe types. In every band I’m in, I want them to be the best possible version they can be. The scenario you describe does not fit that mantra and therefore does not compute for me at least.