If German is not their native language, they might be open to improvement.
Almost any foreigner I have met so far was very interested in improving, given a constructive and non-judgemental approach from my side. A good way to talk about it is talking about your own short-comings in languages foreign to you, how you try to improve and suggest the same to them.
So for example, assuming English is a foreign language for you, you could say something like
"Hey, did you know you can activate the spell checker in Outlook by pressing CTRL-Ä? I always do that for my mails in English, it catches almost all the mistakes I make."
(Please note that I have no idea what program you use or how to activate the spell checker, you have to find out, that was just an example)
Now if they are a native German speaker and are really bad at it, or have a strong dialect that would be considered flat out wrong in Hochdeutsch/standard German, this is more tricky. Almost nobody I have ever met is open for improvements of their native language. The constructive and self-deprecating conversation you can have with a non-native speaker does not really work. You will always come across as the "Besserwisser" and "Meckerer", as they are already doing their best in something they were taught for at least a decade in school and you are still finding mistakes.
There is really no good way to approach this topic.
If you want to be sneaky about it, you can find a reason to forward this mail to their boss. Maybe their boss asks for a status update and you forward the mail with something like "As X explained in their mail to the client, it will be ready for testing next Friday". Then their boss can think about whether the amount of mistakes in a client facing communication is acceptable.
But bringing it up directly or indirectly to your boss is a minefield. The other answer does a good job of cautioning and explaining the obvious dangers, my advice would be to just leave it as it is, since it's not your responsibility.