Hi @HansHa !

He can only add songs from Spotify in Shaker Mix  .
A Mix is a selection of your group's favorite tracks, which can be played across alternative streaming platform if preferred (Spotify, Apple Music and YouTube Music).

here is helpful link

https://support.deezer.com/hc/en-gb/articles/14366017913373-Shaker-

Thanks,

But if he wants to listen to the entire mix he must use a Deezer account or click the Spotify link for every individual song? Or is there another way?

You're welcome  @HansHa !

If he wants listen entire Mix (or Playlist which you can create on Shaker) ,he need use Shaker .
You can together listen tracks from Mix or Playlist (from Shaker) across preferred alternate platforms  (Spotify, Apple Music, YouTube Music).

Thanks Nina 

He can access the shaker by the URL I  have sent him, but when het opens it and tries to play the mix or playlist. This is what he gets:

The butten just leads to https://www.deezer.com/explore/nl/devices/mobile/

@HansHa 

He is probably click on Shuffle play . When you click on Shuffle then is 
written :
The full Shaker experience is available on the Deezer app! Listen to your friends' favourite songs and add yours to the mix. 

But if you click on image of song to play ,then is open tab where you need
Choose your music platform .
Then he can choose Spotify ,and then will be open Spotify where will be that song played .

I think that with Shaker other people who are not on Deezer can compare his music platform with Deezer and maybe can decide to subscribe to Deezer !

I guess you are right. Rather disappointing outcome. Would be nice if Deezer added a feature to transfer the mix or playlist to other musicplatforms through Tune My Music.