As someone who plays competitive sport (tennis) it does depend on the situation and the person.
If you advertise yourself as a "anyone can come and play", no you can't ask someone to leave. Ideally if you're competitive you have more than one team, in different leagues and the bottom one should be about everyone playing as much as possible. However if you're one team then I think you do need to look at how you can use her.
BUT: Sometimes you get someone who is really bad, and doesn't improve because they really don't get how bad they are. I've played with some people who are bad, and not minded one bit because they listen to you when you say "don't worry about X concentrate on Y" or "another time move to XX if that happens" etc. They improve and they can be lovely to play with.
But I've played with people who are actually better at the start. But they don't realise how bad they are and they rush across the net to try and volley (which you know they'll put in the net if they do get to it) which leaves you trying to cover the down the line plus the cross court as they're playing in the middle of the court. That's irritating after the first couple of misses.
Or you say "it's better if you stand here because where you're standing means they just put it to your feet which is hard". And they reply with "oh I'm happier here" so every other shot it put straight to their feet and they miss it.
I don't worry about playing with someone weaker than me at all. It can be quite nice to take the lead. But if they are making the same mistakes again and again it really spoils it, especially if you try helping them and they ignore it.
It's also no fun playing against someone where one of the players if totally out of their depth. Do you hit it at them and get the point, or dolly it over the net to them, or play to their partner? That's not really a fun game for anyone.
I remember turning up to play a team once where the captain had brought his girlfriend into play. I note he chose not to partner her. We lost 2 points in a set against her and partner. And that was with playing on the partner just to get a hit of the ball. She missed every ball that went to her, and poached terribly too. At the end one of the team asked if she'd enjoyed the match. "Oh yes," she said, "I hope to play every match."
She clearly didn't realise how much weaker she was than everyone else.
Now if she'd said "I loved it, but I'm going to have to do a lot of practice to get good enough to play again" then I think she'd have had quite a different reaction.