since Transglutaminase is a protein binding enzyme that is made by a yeast like fermentation process from grain matter, this enzyme by itself is kosher in theory, however most would still want the process rabbinically supervised before it's used in food binding and texture applications for the kosher consumer.
Transglutaminase is commonly used to make that red and white imitation crab meat, which are actually pieces of cod bound together into firm rods for sushi making applications. Since this item is commonly found in kosher sushi, the Transglutaminase used to make it must be kosher.
I recall talking to a mashgiach at one of those vegetarian fake meat and fish kosher restaurants, telling me the biggest headache he has is with the flavoring, texture and coloring agents used to make the same piece of plain bean curd or mushroom, take on the desired appearance flavor and texture.
The best way your so-in-law can be sure the Transglutaminase he uses is kosher enough, would be to contact the maker or distributor and figure out what other well certified kosher commercial products it is being used, and confirm it's use with that product maker's rabbinical supervision.
I believe the Satmar hasidic sect are particularly involved with commercial ingredient kosher certification for most of the major food processors here in the US, so perhaps contacting them would also give you the best answers. I know a family in the grocery wholesale business who has contacts with such Satmar's.
Most "meat glue" used in the USA comes from this one company,
so perhaps contacting them would calm your Kosher concerns;
many a hot dog and chicken nugget owes it's cohesion
to this marvelous protein binding enzyme.