Does a PhD student need to focus more on working in theory, in order to have a real chance of becoming a professor?
I agree with Buffy's answer, but also want to expand on what knzhou alludes to in a comment: STEM has many sub-fields, some of which are more theoretical or applied than others. As it turns out, the walls separating these different fields are far from impermeable. Perhaps your friend's PhD really is too applied for his current department, making a tenure case difficult in that field? Perhaps their research would be a more natural fit for another department, or a national laboratory? For example, there are plenty of people with physics PhDs that have ended up as tenured professors in various engineering departments.

Now, if the research best fits departments with the current name, then your friend might just have to go down the status ladder a little bit to have a realistic chance of tenure. For example, an expert in a field-specific numerical method can be seen as too much of a one-trick pony for the absolute top institutions.

The bottom line is this: if your friend enjoys their research there are a lot of possibilities to explore before giving up on it completely. They really should talk to their mentor(s) to get personal career advice.