You shouldn't think what any chemicals you are using are you should know.
You friend has a duty of care to the customers to only use safe, legal chemicals and to use them appropriately. He also has a duty of care to you not to put you at unnecessary risk. If his, or your, actions cause a problem and things have not been done properly then professional indemnity insurance will not pay up.
Both of you do have professional indemnity insurance?
Gallup XL is just glyphosate at 36% concentration. I'm not an expert but my understanding is that glyphosate at that concentration is only supposed to be sold to, and used by, people who have the appropriate training and equipment.
Glyphosate sold for non-commercial use is much less concentrated, maybe 5%.