Is it illegal to give a friend a tattoo without a license?

Yes, depending on the state.

JJ:

yes it illegal but depends on your friends age if they like 18+ and if they fully consent to it they assure you they wont complain or sue you then its there own grave if they dont like it

But technically IT IS ILLEGAL

but i will say this if a friend is old enough understands what your gonna do its there own stupid fault

Never tatoo someone while there drunk or sleep cos that will look like you took advantage of your friends. you must be sure they want it, tatoos aint easy to remove :(