Is it bad to drink boiled water mixed with normal cold tap water?

So when I drink tea I usually add some milk, but when we’re out of milk I use cold water so I can drink it immediately, and my boyfriend told me, and was really serious about it, he really thinks it’s bad for your health to mix boiled water with normal cold tap water. He said that you can get diarria, srry don’t know what else to call it, upset bowels the next day.

I mean I live in Holland the water here, and in Germany, is 99,99% clean. In Hungary, Polen, Formal Sovjet-Union, the water isn’t as clean and you’re tought to not drink tap water. But here it’s perfectly safe I imagine?