Find out what their software development budget should be, subtract from that what it actually is. That figure is probably their advertising budget.

Joking? Maybe. Maybe not.

Their financial docs include a line item for advertising and it's even broken down by marketing segment - outdoors, automotive, aviation etc.
https://www.garmin.com/en-US/investors/sec/


I'll leave it to your friend to dig into the docs and see if there is a further breakdown somewhere as to the type of advertising.