“就好” in this sentence imply that the speaker is perfunctory response to you, due to your complaining something about a female, actually the speaker don't care pretty much. E.G : Mike said "My girlfriend should be getting angry, unless I wore her high heels shoes running around the playground .It's ridiculous, isn't it?" But Benjamin neither wanna comment it ,nor cares your negative emotion,so he said"Mmh... Whatever! As long as she is happy" （I.E 你让她高兴就好） “就好” in this sentence is more closely to " Whatever!"

nǐ chī le jiù hǎo 。 你 吃 了就 好。 As long as you have eatten, it will be nice tā ɡāo xìnɡ jiù hǎo 。 他 高 兴 就 好 。 As long as he is happy, it will be nice nǐ huí lái le jiù hǎo 。 你 回 来 了 就 好 。 It will be nice as long as you come back

It's like 'That's it'.

Nǐ ràng tā gāoxìng, jiù hǎo ba! 你让她高兴，就好吧！ This sentence doesn't make too much sense. However, if it is 你让她高兴就好, it sounds like a decent sentence. It means "It would be nice if you can make her happy" something like that.