When in doubt, the first thing I do is often give loved ones protective charms and stones.
Maybe get a protection sigil engraved on a tag for his collar. Perhaps the Algiz rune ᛉ, a pentagram ⛤, or other symbol of protection.
Here’s an easy guided Pet Protection Spell, and @SilverBear recently shared her version of a Protection Spell for her Dog! 
Also, make sure that your friend is okay with you casting a spell on her dog.
Good luck and many blessings! 