If you want an informal idiom, there is don't get your panties in a twist:
[The Free Dictionary]
get (one's) panties in a twist
To become overly upset or emotional over something, especially that which is trivial or unimportant. Primarily heard in US, South Africa. Ah, don't get your panties in a twist, I'll have the car back by tomorrow morning! In my opinion, people are getting their panties in a twist over this election.
The UK equivalent replaces panties with knickers:
If someone is getting their knickers in a twist about something, they are getting annoyed or upset about it without good reason.
[British, humorous, informal]
She didn't know why he was getting his knickers in a twist.
As with the other answer, I would not recommend saying this to someone unless you're sure they won't take offence to it.
The more neutral way of responding in this situation is not to use an idiom at all, but to simply say, "I think you're overreacting," "You need to calm down," or, if you want to avoid any kind of judgment of the person at all, just, "I don't think it's that bad."