I wonder how accurate your (friend's) dosage is. 10-20mg is a LOT for buprenorphine
To answer your questions, no, buprenorphine isn't harmful if used infrequently. How addictive it is is subjective, like almost anything-- if you're prone to getting addicted to things, than you can definitely
get addicted to this. If you're one of those people who can pick things up and put them right down again (I don't know any people like this in real life, but I've read/heard they exist), then you'll probably be alright if you ONLY use once/twice a week.
Also, don't forget there's a big difference between being psychologically and physiologically addicted to substances. Using once a week won't get you physiologically addicted, but the psychological side can be a little trickier.