I am not a very funny guy, but you can tell your friend this true story:
I was 13 and, together with my family, was visiting friends of my parents in Fairford one year. The man had lost an arm and a leg in WWII. Of course, I decided I wanted to entertain everyone at the table and started telling the following jokes:
What do you call a man with no arms and no legs on your doorstep? Matt
What do call a man with no arms and no legs in your swimming pool? Bob
Etc., etc.
Not once did it dawn on me who my audience was. Then my brother kicked me under the table.
I wanted to die on the spot.