It depends on which internal HDD his MBP has. If it’s ssd chanches are that is way faster of any external solution.
If he has enough space on it he should be able to keep the LR catalog on the internal HDD and the files on the external one.
It might even deepend on which version of LR he’s using (in the past few months Lightroom Classic CC has been optimized and is way faster than before, as far as i can tell).
Another trick might be to use another external ssd as a repository for LR’s cache.