On the face of it this doesn't make sense. If the field in question is a foreign key field with a dropdown list attached, why would anyone need to type directly into the field? Surely if they did, and typed anything other than a foreign key, this would be counter to purpose.
If users typing directly into this field is expected and common behaviour (you say "this works fine for other users" whatever "this" is) it suggests possible poor practice—at very least you should probably have validation on that field set to only accept values from the attached list. If not, then the problem you describe could well arise from that particular user have a muddled understanding of what to do.
And what happens when you press a key with such a value list deployed isn't exactly what I'd call "type ahead". Generally, you can scroll the field to first displayed value that starts with that first letter or a few letters if you type fast.
I believe that there are behavior differences depending on OS platform and FileMaker version so I'd check those first. You might then check any keyboard settings in OS control panel to see if there's something different.
(And when I need such a type ahead capability, I either use a different set up that supports an auto-complete enabled value list where the 2nd field is now the first field--with scripted support and a different relationship used to look up the ID and manage cases where there is no matching name or multiple matching names, or I use a filtering selection portal that updates with each letter entered into a search field above the portal.)