I dont think Kahga needs to offer another tadpole "solution" or be told about it. We are already telling about our tadpole to way too many people as it is. We dont need another false hope or giving that info to people we dont know. (Unless its done as a obvious mistake with bad consequences later on)
The evil players do not need to become buddies with every other evil character or plot. In fact, being evil would work against that.
Kahga, being a Shadow Druid would consider everyone else the enemy and the player would discover that and her situation then needs to be solved along those lines. Even if the player is playing evil Kahga and Shadow druids would be competition, not allies.
This would also remove the "im evil lets just kill everyone muahahaha" evil gameplay a lot of players are already complaining about, and give them some more interesting options, gameplay and consequences.
Giving Kagha a means to aid the player would reduce the "maniac evil PC" problem because if she had a solution then siding with her would be a choice that would make sense for a self interested, rational, but ultimately amoral person to choose.
As is she offers nothing. You as the player are looking for a way to fix the tadpole problem. She asks you to get the tieflings to leave. Why would you do this unless you just hate tieflings? Generally you wouldn't because there is nothing to be gained for risking your life. But if she could offer something the PC wants while the tieflings don't an evil character has a strong reason to consider her offer and do something horrible for their own gain.
This also enriches the choice to side with the tieflings because it means your good aligned character is turning down a potential way to save their own life because they refuse to do something morally repugnant.
It adds nuance to the choice.
As for the consequences of choosing the evil option of siding with Kagha? I assume how you handle this will influence Act 2 when you run into the tieflings again in Baldur's Gate. Or don't as they're all dead.
By and large that should be how the good vs evil choices are handled. Evil gives immediate, short term gains but cut out options later in the game because of your actions while good choices are crippling in the short term due to your sacrifices but have big pay offs later in the game when you run into people you've helped before.