If you are interested in buying, it is best to bring an appraiser with you to the second viewing. They can then tell you how high the effort and renovation costs will be.
I spoke with an expert today and sent him a picture of the damage. He said it's not a big deal. They would remove part of the wall from the inside and then fill it with resin. Cost around €500 (which I actually think is quite cheap). Nevertheless, I would rather do something on the outside wall based on my feeling. I mean, the crack actually appeared there.
I would be even more cautious with such obvious damages. One might be tempted to overestimate the obvious and overlook what is hidden. Find a professional if the house is of interest to you. But not just to assess the crack, rather to check the entire property. Who knows what else might be hidden there.
Your expert sounds like a craftsman who can cover something up. The users here are talking about [Sachverständigen] who know how to assess buildings and their condition.
The professional "expertise" is very interesting...
So the effect is eliminated but not the cause...meaning the wall remains damp...it just doesn't rain in anymore...
As already written, also for you a company that knows about drying out...something like this needs to be excavated from the outside and sealed...and an expert for the rest of the house would be advisable as suggested by other users...