Hi
I am seriously politically uncorrect
He has a girlfriend? Could be worse, he could have had a boyfriend.
Why bother teaching English to Germans? Just tell them to read some 60's English comics, they'll soon get the drift. Amongst all the 'Donner und Blitzens' your Teutonic student will learn such helpful phrases as 'Die German Pig' and 'Kill das Englander' not to mention 'You will talk, Tommy, after I've finished with you'. Pukka stuff
As for your erring hubbie, try this. Take a sharp Stanley knife, carfully make a slit in the packaging for his condoms and remove them. Fill several of those latex objects with oven cleaner. Carefully replace them and superglue packaging shut.
Next time he uses one, guaranteed to make his eyes water
Another good solution is cut both his handbrake cable and his brake pipes. Yay!
Or, ok, sensibly - a private detective.
The name's Marlow, baby, and I don't come cheap